Wrap fish in foil with small amount of butter. Bake in 350 degrees oven until almost cooked (5 to 7 minutes). Make white sauce on range top by melting butter and adding flour mixed with seasonings, stirring constantly until smooth and well blended. Pour in milk slowly, stirring constantly. Bring to boiling point and boil for 2 minutes. Place fish in baking dish and pour on white sauce. Heat in oven until ready to serve.