Rinse fish thoroughly in cold water, and pat dry. Sprinkle fish with salt and pepper; place in a well-greased 12- x 8- x 2-inch baking dish. Sprinkle onion evenly over top of fish. Melt 1/4 cup butter, and pour over fish. Add wine to baking dish.
Bake, uncovered, at 350° for 20 minutes or until fish flakes easily when tested with a fork, basting occasionally with pan drippings. Remove fish to a serving platter, and keep warm; reserve pan drippings.
Place reserved pan drippings in a medium saucepan. Cook over medium heat until mixture is reduced by half; remove from heat. Stir in remaining butter, chopped parsley, and Hollandaise Sauce; mix well. Pour sauce over fish; garnish with parsley sprigs, and serve immediately.