Q) I cooked fish in a meaty oven. Am I now allowed to heat up the fish in a milky toaster oven?
A) The first potential issue that needs to be addressed is the combination of fish and meat
The Shulchan Aruch (YD 116:2) tells us that one needs to ensure not to eat meat and fish together because it is conducive to contracting tzora’as (leprosy).
Therefore, if there is meat residue or gravy on the baking tray and one now cooks fish directly on the same tray the fish will be subject to the problematic combination of fish and meat and may not be eaten. (If the fish contains sixty times the volume of the meat residue, it is possible that the meat residue is nullified[1]. A Rav should be consulted.)
If the baking tray and oven[2] are clean and the fish is cooked directly on the tray, the next issue that must be clarified is the bliyos of meat that will now be transferred into the fish. (Bliyos is the term used to describe food matter that has been absorbed into a utensil.) Do bliyos pose a problem of sakono, or is it only the actual tangible substance of fish and meat that create a dangerous combination?
The Taz (YD Siman 95 s.k. 3) maintains that bliyos do not pose a problem of sakono. Therefore, as long as the oven tray was clean of meat residue, there is no problem of sakono and the meat may be eaten.
We now need to address the next aspect of the question: can a parev food item that was cooked in a (clean) meaty oven be subsequently heated up in a (clean) milky oven?
The situation over here is classified as nat bar nat. Namely, when a parev food item is cooked in a (clean) pot that was previously used for either meat or milk; the utensil is classified nat [abbreviation for nosein ta’am – meaning that it contains primary bliyos of the food that was cooked inside it]. The parev food cooked inside the utensil is classified nat bar nat [abbreviation for nosein ta’am bar nosein ta’am – i.e. it contains secondary bliyos imparted from the original food into the utensil; and then from the utensil into the parev food].
The Shulchan Aruch (YD Siman 95 Se’if 2) rules that nat bar nat may be eaten with the “opposite” type of food. For example, if potatoes are cooked in a clean meaty (ben yomo) pot they may be eaten together with cheese (l’chatchilo). [However, even according to the Shulchan Aruch one may not initially cook parev food in a meaty pot intending to eat the parev food together with dairy[3].]
The Remo (ibid) rules regarding nat bar nat that it may not be eaten with the “opposite” type of food. I.e. if potatoes are cooked in a clean meaty (ben yomo) pot they may not be eaten together with cheese (l’chatchilo; however if already mixed the Remo is lenient and allows the food to be eaten). However, even the Remo who is stringent regarding nat bar nat allows it to be placed in a utensil of the other “type” (ibid). Since the Remo does not differentiate between different levels of keilim (i.e. kli rishon or kli sheni) it appears that it is even permitted to cook the nat bar nat food in a kli rishon.
Therefore, in this situation where the fish was cooked in a meaty oven, the fish has the status of nat bar nat and although it should not be eaten together with dairy it may still be cooked or reheated in a milky oven.
[1] ע’ ט”ז סי’ קט”ז ס”ק ב’ ופ”ת יו”ד קט”ז ס”ק ג’
[2] שאם התנור מלוכלוך מבשר, יכול להיות העברת טעם מבשר לדג דרך זיעה בתנור, ואכמ”ל. ויש לעשות שאלת חכם
[3] ש”ך שם ס”ק ד’