Spread the onions and pepper in a 13 by 9-inch glass baking dish and place the fish on top. Dot the fish with butter.
Sprinkle with a little Worcestershire sauce.
Bake for 12 minutes, then baste fish with pan juices.
Sprinkle the fish with Parmesan and then place under the broiler for about 2 minutes or until the cheese browns. To serve, spoon the vegetables over the fish.