Is it allowed to cook fish and meat at the same time, does it matter if they are cooking or warming, which sefer speaks about this, and is there a difference for Ashkenazim and Sefardim?
ב"ה Shalom, The Gemara in Pesachim (76b) states: “Regarding fish that was baked in an oven together with meat, Mar bar Rav Ashi said, one may not eat it, for this (eating fish and meat together) causes leprosy.. Following the Gemara, the Shulchan Aruch (Yoreh De’ah, Chapter 116:2) rules that one may not eat fish and meat together because of the danger involved. The Rema adds, that neither may one bake meat with fish, however after the fact, if they had been baked together they may be eaten . Although, only the Rema speaks about baking, it is not in disagreement with the Shulchan Aruch, so therefore there is no difference on this issue between Ashkenazim and Sefardim and both Ashkenazi and Sefardi Rabbis are of the opinion that one may not cook, bake, roast, or eat fish and meat together. Therefore, if you meant by " to cook fish and meat at the same time", that they are in the same pot at the same time it is clearly forbidden. However, one may fry or cook fish in a clean meat pan or vice versa. All the best