This is a wonderful recipe I've had for over 50 years, so long I don't even remember where I got it. When we were first married, in 1958, fish and seafood were very inexpensive, so we had it often. Our four girls all loved this casserole and would ask for it quite often. It is a wonderful comfort food and makes a great dish for company.
Preheat oven to 300 degrees. Butter a 3 quart casserole.
Simmer fish in small amount of water until tender. Flake into large size pieces(4 cups). Saute' mushrooms in 2 Tbsp. butter about 4 minutes.
Melt the remaining 2 Tbsp. butter in saucepan, stir in the 2 Tbsp. flour, mixing to combine. Slowly whisk in the milk, stirring constantly. Add cheese, stir until cheese melts. DO NOT LET BOIL. Add Worcestershire sauce, salt, pepper, sherry, fish and mushrooms.
Turn into the buttered casserole. Top with the buttered bread crumbs. Bake at 300 degree for 30 minutes.