Melt butter and fat in double boiler; stir in flour and pepper; stir in milk gradually.
Cook until smooth and thickened.
Combine 3/4 of the crushed potato chips, the fish, mushrooms, and the sauce.
Pour into a greased 1 1/2 qt casserole; top with the rest of the crushed potato chips.
Bake in a moderately hot oven of 375°F for 30 min.