Rinse fish thoroughly in cold water; pat dry. Place fish in a 13- x 9- x 2-inch baking dish. Sprinkle with salt, pepper, and Worcestershire sauce. Arrange onion and potatoes over top; dot with butter.
Cover and bake at 350° for 45 minutes or until fish flakes easily when tested with a fork. Garnish with lemon slices; serve hot.