If one cut an onion [or any other Charif] with a meat knife and then cut the same onion with a dairy knife, what is the status of the onion and dairy knife?

Background:[1] Sharp foods, such as onions and garlic^[2], have ability to absorb the taste absorbed in a knife. Thus, if an onion was cut with a meat knife [that has been used to cut hot meat, or is washed with meat dishes in hot water[3]], the entire[4] onion absorbs the meat taste and becomes meaty. This applies even if the knife was not Ben Yomo at its time of use.[5] We will now analyze the law regarding if both a meat and dairy knife were used for the same onion:

The status of the onion: If the onion that was cut with a meat knife was then cut with a dairy knife [that have been used in the past to cut hot meat/dairy] then the onion is forbidden, as it has absorbed from both meat and dairy. This applies even if both knives were clean and not Ben Yomo.[6] [However, if the meat or dairy knife have never been used to cut hot meat/cheese, and have not been washed with hot meat or cheese, then the onion remains Kosher, and requires a mere rinsing.[7]]

The status of the dairy knife: If the meat knife has been used in the past for cutting hot meat, or being washed with hot meat, then the following is the law regarding the dairy knife: Some Poskim^[8] rule the dairy knife is forbidden and must be Koshered.^[9] Other Poskim^[10], however, rule the knife remains Kosher.^[11] [Seemingly, one may be lenient if the original knife was not Ben Yomo, or if the Charif was cut on its other side, beyond the Netila area of the original cut, to permit the knife without Koshering.[12] Nonetheless, it is best to Kosher it being that doing so is so easily attainable. One can simply place the knife over a flame and have it Koshered through Libun Kal.]

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[1] Michaber and Rama 96:1-2

[2] Michaber 96:2; Admur 447:40

[3] Beis Yosef 96 in explanation of Rambam; Beis David Y.D. 38; 39; Zivcheiy Tzedek 96:34; Kaf Hachaim 96:48

[4] Rama 96:1; Peri Toar 96; Rashba Toras Habayis Hearuch 4:1, brought in Tur ibid; Ran Chulin 41a

Opinions in Rishonim who hold only Netila is required: The following Rishonim explicitly rule that one is only required to remove a Netila’s worth: 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]

[5] 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

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]

Custom of Sefaradim: 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. “Gimel Midos Besakin”]

[6] If the onion was cut from opposite sides: If the onion was cut from opposite sides, two Netila’s distances [4 cm] from each other, then seemingly, Bedieved the onion remains Kosher, and each half [Netila area by each side] becomes dairy or meaty. Furthermore, even if one cut the onion on the Netila side, seemingly past the Netila area remains Kosher as we rule Bedieved like the opinion who rules that Charif does not absorb more than Kdei Netila. [See Rama 96:1] This certainly applies according to the Sefaradim who never prohibit more than a Netila. Vetzaruch Iyun.

[7] Beis Yosef 96 in explanation of Rambam; Beis David Y.D. 38; 39; Zivcheiy Tzedek 96:34; Kaf Hachaim 96:48

[8] Magen Avraham 451:31; Chochmas Adam 49:10; Implication of Admur 451:54 that Charif transfers taste to the knife; See also Even Haozer 96:3

[9] The reason: As the Charif has power to absorb the meat taste into the dairy knife.

[10] P”M 447 M.Z. 13; Daas Torah 96 p. 258

[11] The reason: As a knife which cut a Charif only has ability to transfer taste to the Charif and does not have ability to extract its absorbed taste into itself. [P”M ibid]

[12] The reason: As this is a case of Sfek Sfeika, as some Poskim rule non-Ben Yomo knives do not prohibit a Charif, and other Poskim rule that knives do not absorb from the Charif. Certainly, if one cut the onion from opposite ends, in which case we rule that Bedeived the onion did not absorb past its Netila worth, that the knife does not need to be Koshered.