What is the law if one cut onions with a dairy knife and fried them in a meat pot?
If a Pareve Charif/sharp food was cooked in a meat pot then even if the pot was clean and not Ben Yomo, the food becomes meaty and is forbidden to be eaten with milk. Likewise, if a Charif [i.e. sharp] food, such as onions and garlic, was cut with a meat or dairy knife [that has been used in the past to cut hot meat/cheese], then the entire food becomes meaty/dairy and is forbidden from being eaten/cooked with the opposite food. This applies even if the knife was clean and was not Ben Yomo [i.e. had not cut hot meat or cheese within the past 24 hours] at its time of use. Accordingly, if an onion was cut with a dairy knife that has been used in the past to cut hot dairy, the entire onion absorbs the dairy taste and becomes dairy. If one later fries this onion in a meat pot, then the onions extract and absorb the meat taste in the pot, thus making it forbidden due to a mixture of meat and milk. This causes that everything becomes forbidden, including the onions, pot and any utensils used for mixing the food.
What is the law if one cut onions with a dairy knife and cooked them in a meat pot together with water or other foods?
If one did not fry the onion first, but initially cooked the onions together with other foods in the meat pot [i.e. in a soup or Chulent], then if that food contains 60x versus the onion, or 60x versus the blade of the dairy knife, then everything is permitted, including the onion. This applies even if one cooked the onion in chicken soup, or a meat cholent. If, however, the food does not contain 60x the onion, or the blade of the knife, then if the pot or spatula was Ben Yomo of meat use, or there were meat ingredients in the pot together with the onion, then everything is forbidden [i.e. the pot, soup, and spatula]. If, however, there were only Pareve ingredients cooking in soup, and the pot and spatula was not Ben Yomo, then if the onion is a minority ingredient of the mixture, then the food remains permitted [and becomes dairy], although the pot is forbidden and must be Koshered. If the onion is the majority ingredient in the food, then everything is forbidden, even if the pot was not Ben Yomo. If one first fried the onion in a meat pot and then mixed it with other foods [whether Pareve or meaty/dairy], then in all cases, the other foods must have 60x versus all the onions, otherwise everything is forbidden.
Fried: If one fried onions that were cut with a dairy knife [that was used in the past to cut hot dairy] in a meat pot, everything is forbidden [the pot, onions and spatula].
Cooked with other foods: If one cooked onions that were cut with a dairy knife [that was used in the past to cut hot dairy] in a meat pot together with other foods, everything is forbidden [the pot, onions and spatula], unless the food contains 60x versus the blade of the knife [or onions, whatever is less], or the food is Pareve, and the pot is not Ben Yomo, and the onions are a minority ingredient, in which case only the pot is forbidden while the food remains Kosher.
A word of advice regarding Charif:
Due to the complex Halachic issues that a Charif vegetable poses if it is cut with a meat or dairy knife [and then gets used with the opposite food or vessel], it is highly recommended that every kitchen establishes a rule that all Charif vegetables be cut with a Pareve knife and Pareve cutting board. Experience has shown that leaving a meat or dairy cut Charif vegetable around the fridge is bound to create problems, as people forget its status and can come to use it for the opposite food.
 Rama 95:2; Implication of Aruch 24; Rashba 449; Admur 447:45; See also Michaber 103:6; Pischeiy Teshuvah 96:4
Other opinions-Ruling of Michaber: Some Poskim rule that according to the Michaber [mentioned in 96:1 regarding a knife which cut a Davar Charif that if it was not Ben Yomo it remains Pareve according to the first opinion then] if the pot was clean and not Ben Yomo, Bedieved if it was cooked with the opposite food it does not require nullification. [Hakashrus 10:113; See Kaf Hachaim 96:11] Other Poskim argue on the Rama’s conclusion based on Rashba and Aruch ibid and suggest that even according to the Rashba and Aruch who rule regarding a knife that Charif turns non-Ben Yomo into Ben Yomo, by a pot the rule is different and Charif does not make non-Ben Yomo into Ben Yomo. The reason is because the concept of Nat Bar Nat still applies even by Charif, and hence only when there are two Nats, such as by a knife, do we say the taste is Beiyn, however when there are three Nats, such as the case in the case of a pot, then it is permitted. Practically, this opinion concludes that by definite Charif one is to be stringent like the Rama while by questionable Charif one may be lenient. [Makom Shmuel 89, brought in Pischeiy Teshuvah 95:4]
Is the concept of a Davar Charif a Biblical or Rabbinical concept? See Chapter 5 Halacha 3!
The reason: As the sharp food enriches the meaty taste of the pot and effects that its taste be considered Beiyn [direct taste from meat] and is thus no longer considered Nat Bar Nat. [Shach 95:7; Admur ibid that it becomes Tam Rishon]
 Michaber 96:2; Admur 447:40 [Rav Yaakov Yosef ruled on one occasion that the onions of today are no longer considered Charif and one may hence be lenient. I have not found this opinion in other Poskim.]
 Beis Yosef 96 in name of Rambam; Beis David Y.D. 38; 39; Zivcheiy Tzedek 96:34; Kaf Hachaim 96:48; See Q&A!
 Rama 96:1 in final ruling that so is the Lechatchilah custom; Peri Toar 96; Rashba Toras Habayis Hearuch 4:1, brought in Tur ibid; Ran Chulin 41a
The reason: The reason behind this opinion is because they hold that all spicy foods have the same status as a Kurt Shel Chalatis for all matters and hence they absorb the taste of the knife throughout their entire mass. [Taz 96:7; Lechem Hapanim 96:11; Beis Lechem Yehuda 96:9; Kaf Hachaim 96:25]
Other opinions who hold only Netila is required: Some Poskim rule that one is only required to remove a Netila’s worth from the Charif and the remainder is Pareve. [Michaber ibid; Raavad Avoda Zara 76a, brought in Rashba ibid; Semag Lavin 140; Reah in Bedek Habayis 4:1, brought in Ran Chulin 41a; See Beis Yosef 96:1]
The Sephardic custom: The Sephardim follow the ruling of Michaber that a Davar Charif only absorbs a Netila worth even when used with a dirty Ben Yomo knife. [Erech Hashulchan 96:2 that so rule majority of Poskim; Zivcheiy Tzedek 96:4 and 21; Kaf Hachaim 96:4 and 27] However, the Peri Toar rules to be stringent like the Rama to forbid the entire Charif.
 The reason even a clean knife transfers taste: As the sharpness of the food and the pressure of the knife together causes the Ben Yomo taste that is absorbed within the knife to get extracted and absorbed within the food to the point of a thumbs width. This absorption is of such good quality that it is considered as if it became directly absorbed from the actual meat and is hence not defined as Nat Bar Nat. [Shach 96:2; Kreisi 96:1; Peri Toar 96:1; P”M 96 S.D.2; Chavas Daas 96:1; Chochmas Adam 49:1; Beis Yitzchak 96:2; Aruch Hashulchan 96:5; Zivcheiy Tzedek 96:1; Kaf Hachaim 96:1] Now, if the knife is not Ben Yomo but is dirty with fat, then the fat becomes absorbed into the food due to its sharpness and pressure of the knife. [Taz 96:1]
 2nd opinion in Michaber 96:1 regarding forbidding Netila [omitted from 96:3, 103:6, 114:8]; Sefer Hateruma 60, brought in Tur 96:1; Shach 96:6 and 19 “So is the custom, and so rule all the Achronim”; Admur 447:40 and 55 and 59; Daas Torah 96
The reason: The reason behind this opinion is because they hold that all spicy foods have the same status as a Kurt Shel Chalatis and hence have the ability to extract and enhance the spoiled non-Ben Yomo taste absorbed within a knife. This absorption of such good quality that it is considered as if it became directly absorbed from the actual meat and is hence not defined as Nat Bar Nat. The reason that the quality of taste absorbed is so great is because a) knives commonly have fat residue on them and b) The sharpness of the Charif and pressure of the knife cause a greater quality of taste to be absorbed. [Taz 96:3; Shach 96:6; Admur 447:59; Kaf Hachaim 96:9]
Other opinions: Some Poskim rule that if the knife was not Ben Yomo, and is clean, then one may use the spicy food for the opposite food. [Stam opinion in Michaber 96:1; 3; 103:6; 114:8; Maharam Teshuvos Upesakim 22, brought in Tur 96:2 and Rosh Avoda Zara 38] The reason behind this opinion is because they hold that only a Kurt Shel Chalatis is a true Charif, as it can puncture the intestines of a cow which eats it, and hence even when cut with a clean and non-Ben Yomo knife, it has ability to extract and enhance the taste. However, other sharp foods, are not sharp enough to extract and enhance non-Ben Yomo taste. [Taz 96:1]
Custom of Sephardim: The Michaber ibid does not arbitrate between the two opinions that he records. According to the Kelalim of ruling of the Shulchan Aruch, whenever the Michaber writes like one opinion and then brings a second stringent opinion, his intent is to rule that one is initially to be stringent like the second opinion, although one may be lenient in a case of great loss. Hence, if the Charif was cut with a clean non-Ben Yomo knife, a Netila worth must be removed and 60x is required, although in a time of great need or loss one may permit the food even if it does not contain 60x. The reason the Michaber omitted this stringency from other areas [see 103:6; 114:8] is because he relied on the fact that he already recorded their opinion here. [Kaf Hachaim 96:10, 11, 50; See Shach 96:19; Taz 96:10] However, other Poskim learn that the Michaber in truth is lenient. [P”M 10 S.D. “Gimmel Middos Besakin”]
 When measuring versus the onion, does one measure versus the entire onion or only a Netila worth of the onion? If the Charif food was sliced in half then one only requires 60x versus the Netila/finger area of the Charif food [i.e. 2 cm of each side of the slice ], or 60x versus the blade of the knife, whichever is less. [Michaber 96:1; Rama ibid that Bedieved we are lenient like Michaber; Peri Toar 96; Raavad Avoda Zara 76a, brought in Rashba ibid; Semag Lavin 140; Reah in Bedek Habayis 4:1, brought in Ran Chulin 41a; See Beis Yosef 96:1] If, however, the spicy food was cut into small pieces [less than a Netila worth-2 cm-each ] then according to all opinions if one went ahead and cooked it with the opposite food, one requires 60x versus the entire spicy food or 60x versus the blade of the knife, if it is of smaller size. [Rama ibid; Shach 96:9; Kneses Hagedola 96:34; Peri Chadash 96:8; Kreisi 96:6; Lechem Hapanim 96:10; Beis Lechem Yehuda 96:8; Chochmas Adam 49:3; Kaf Hachaim 96:22]
 Does one measure against the blade or the onion? If one cut a sharp food with a meat knife and cooked it together with milk, one needs to measure 60x in the dairy food versus the Netila area of the food, or versus the knife which he used to cut with. One can choose to measure against whichever amount is less; if the area of the blade is less than the Netila area, he may measure versus the blade; if the Netila area of the food is less than the blade one may measure versus the Netila. [Michaber 96:1; This applies even according to the Rama as the food has absorbed Heter and not become Chanan.; Taz 96:6; Shach 96:9; Lechem Hapanim 96:9; Beis Lechem Yehuda 96:7; Chavas Daas 96:5 Biurim 1; P”M 96 M.Z. 6; Kaf Hachaim 96:21]
How much of the blade is one to measure against? If one is unsure how much of the blade was used to cut with, one is to measure 60x against the entire blade. [Shach 96:8 Taz 96:4; Rashal Gid Hanashe 42; Kol Habasar 62; Kneses Hagedola 96:33; Minchas Yaakov 61:7; Lechem Hapanim 96:7; Beis Lechem Yehuda 96:5; Chavas Daas 96:4; Chochmas Adam 49:2; Kaf Hachaim 96:17]
Measuring versus the amount of dairy the knife absorbed: In all cases that one is aware of the amount of hot dairy that the knife was used for cutting since its purchase, then one may measure versus the meat if it is a smaller amount than the blade or food. [Bach 96; Zivcheiy Tzedek 96:19; Kaf Hachaim 96:124]
 Taz 96:5; Shach 94:23; Maharam Melublin 28; Admur 447:60; M”A 447:38; Kneses Hagedola 96:2; Peri Chadash 94:21; Minchas Yaakov 61:6; Beis David Y.D. p. 16; Lechem Hapanim 96:7; Beis Lechem Yehuda 96:5; Chavas Daas 96:5 Biurim 1; Erech Hashulchan 96:5; Kaf Hachaim 94:60; 96:19
 See sources in previous footnote!
 See Rama 95:2; Taz 103:9 that if the Charif is a minority ingredient, then the mixture is not considered Charif. See next footnote
 As we consider it that the pot has absorbed the dairy taste carried in the onion, and it is hence similar to one who cooked dairy in a Non-Ben Yomo meat pot in which case we rule that the pot is nevertheless forbidden. [See Michaber 93:1; Shach 93:3 based on Rama in 94:5; Beis Lechem Yehuda 93:4; Chavas Daas 93:3; Kaf Hachaim 93:16; 94:54] Now, although the onion here is in minority, it is only its Charifus, and ability to extract non-Ben Yomo meat taste from the pot, which is negated, and not its ability to transfer taste into the pot. It is also similar to the case that a potato from a meat cholent was cooked in a dairy pot with other foods, in which case we rule that if there isn’t 60x versus the potato, the pot is forbidden even if it is not Ben Yomo. [See Rama 94:5 as explained in Shach 94:18; Taz 94:8; Toras Chatas 57:14; Chavas Daas 94:16; P”M 94 S.D. 18; M.Z. 7; Biur Hagr”a 94:18; Kaf Hachaim 94:52; See the following Poskim that by food we do not say Nat Bar Nat: Shaareiy Dura 60:3; Kneses Hagedola 95:26; P”M 93 S.D. 4; 94 M.Z. 1; 95 M.Z. 1; Chavas Daas 95:2; Kehilas Yehuda 95:1; Zivcheiy Tzedek 95:8; Kaf Hachaim 95:10; All the more so does this apply in this case, where we view the Charif as Beiyn of dairy.]
 See Rama 95:2; Taz 103:9 that if the Charif is a majority ingredient, then the mixture is considered Charif and hence extracts the meat taste from the pot, thus making everything forbidden.
 In such a case the onions become Chanan and it does not suffice to measure 60x versus the blade of the knife, or versus a Netila’s worth of the onion.