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. If one did not fry the onion first, but initially cooked the onions together with other foods in the meat pot [i.e. chicken soup], then if that food contains 60x the onion, everything is permitted. Furthermore, even if it does not contain 60x, if the onion is a minority ingredient everything is permitted, as the general food is not considered sharp.
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
 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”]
 Rama 95:2; Taz 103:9