Preheat oven to 350 degrees F.
Season the fish with the salt and pepper. Spread the onions and pepper in a flameproof 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.