Preheat the broiler. Season the fish with salt and cayenne. Place in a shallow baking dish. Combine the butter, lemon juice, and Worcestershire sauce. Pour over the fish. Boil for about five to six minutes, and then turn the fish over with a spatula. Scatter the onion slices evenly over the fish and broil for about five or six minutes more. Watch carefully so as not to overcook. The fish is done when it flakes easily with a fork. Serve with pan juices.
Makes 6 servings