Can someone fry fish in butter?
Answer:
Yes, there is no problem (for Ashkenazim) to fry fish in butter for those who are not particular for eating fish with milk.
According to some Sefardi customs, based on the Beis Yosef (Yoreh De’ah 87:5), one should refrain from this because of health concerns of mixing fish and dairy.
However, the Rema writes that this does not have a clear source, and the common custom for Ashkenaz Jewry is to eat fish with dairy products (such as lox with cream cheese) and not be concerned for the ruling of the Beis Yosef.
Best wishes.
