Kitchen Disasters Class 3: Nat Bar Nat

If the fish was cooked with the meat itself, if would have been forbidden to each with kutach even according to Shmuel. In addition, if one were to place hot kutach in the meat plate itself, it would have been forbidden as well, as it would have absorbed first order meat flavor. However, the plate is not the meat itself, and thus is the first order flavor, allowing the fish to become the second-order flavor.

Rashi and the Ramban are of the opinion that the principle of Nat Bar Nat applies even in the event that the fish was cooked in the meat tray... The Sefer Ha'Terumos quotes the Ri who distinguished between cooking with a liquid and roasting. When one cooks there are three steps (meat to the dish, the dish to the water, the water to the fish) which make it more lenient.

In the name of Rashi, Rabbenu Yehudah ben Natan (Rivan) his son-in-law explained: "It is specifically "bringing up" that is permitted, but if the [fish] were cooked [in a fleishig dish]—it would be forbidden [to eat with milk spread], for it is considered one flavor." ... From the language of Rashi it implies that there is no difference between "bringing up" and "cooking" [for the fish in the dish], since above about Notein Ta'am bar Notein Ta'am, Rashi says: "If they were cooked with actual meat, or to eat milk in the dish, Shmuel would agree that it's forbidden, for it is transferring primary taste"—his explanation implies that it is cooking with the meat that would be forbidden, but cooking in the fleishig pot would be permitted!

RAMA:

There are those that are strict with roasting and cooking and prohibit second-degree taste (Rivan in the name of Rashi, Aruch 34 quotes the Mordechai and ...) and

the custom is to prohibit it before the fact. After the fact, it is permissible in every way (Aruch). This [prohibition] is only eating it with milk or meat itself, but it is permissible to serve it on their dishes before the fact (Issur v'Heter, Aruch) and that is the practice.

Also, if they weren't initially cooked or roasted, rather they were just on a meat plate, it is permissible to eat them with milk, and the inverse (his own reasoning).

Also, if the vessel that they were cooked or roasted in was rancid - meaning it was not used for cooking food within 24 hours - we are lenient even before the fact to eat them with the other type (Aruch, ibid).

All of this is assuming that the food is not sharp, but if it was sharp - e.g. you cooked sharp things in a meat pot even if it has not been used that day, or you crushed spices with a meat pestle, if you [cooked] it in milk, it's prohibited even after the fact, until there is 60 times the meat that is absorbed in them. ... And in any event, [this is not with regards to] an instance of sharpness on account of a small [amount of] spice in it, only if it is entirely sharp.

The custom is to forbid to do this before the fact, meaning to mix these foods with milk, but if they were already mixed, it is permitted. But to cook them in a meat plate to each them with milk originally is a problem even according to the lenient view of the Mechaber

The Maharshal forbids it even after the fact if the food was roasted (no liquid) on the meat tray.

The Beer Heitiv and the Pri Megadim quote the Sharei Durah who says that the principle of Nat Bar Nat applies only when the flavor is transferred by a dish as opposed to a food.

The Pnei Aryeh disagrees and says that Nat Bar Nat applies as well.

The Chavas Daas writes that at the moment of cooking, Nat bar Nat does not apply. Thus if one is cooking fish in a pot, and some milk lands on the outside of the pot, the flavor transfers straight through the walls into the food, and there is no Nat Bar Nat.