Thaw fish, if frozen. Measure thickness of fish. In a lightly greased 10 x 6 x 2-inch baking dish arrange fish. (For fillets, tuck under thin edges.) Bake in a 450° oven till fish flakes with a fork. (Allow 4 to 6 minutes per 1/2-inch thickness of fish.)
Meanwhile, for sauce, cook onion in margarine till tender. Stir in flour, mustard, and, if desired, caraway seed. Add milk all at once. Cook and stir till thickened and bubbly. Cook and stir 1 minute more. Add cheese and beer. Cook and stir till cheese melts. Spoon over fish.