Combine cheese, dry mustard, cayenne pepper (if using) and Worcestershire sauce in a small mixing bowl. Set aside.
Grease a medium-sized rimmed baking pan with one tablespoon of the butter. Arrange fillets in a single layer in pan. Pour ale or beer over fish. Sprinkle cheese mixture evenly over fish. Melt the remaining 2 tablespoons of butter; drizzle evenly over cheese mixture.
Place in preheated oven and bake for 10-15 minutes, or until cheese melts, coating browns and fish is opaque and cooked through. If the fish is cooked but the cheese still needs a bit of browning, place under the broiler for a minute or two, watching carefully so that it does not burn.
Serve immediately with potatoes and/or your favorite veggie.