Rinse fish thoroughly in cold water; pat dry, and place in a 13- x 9- x 2-inch baking dish. Add 1 tablespoon butter, mushrooms, onion, bay leaf, and thyme. Sprinkle fish with breadcrumbs; dot with remaining butter. Add wine to baking dish. Bake at 350° for 1 hour or until fish flakes easily when tested with a fork. Baste occasionally with pan juices.
Remove fish and vegetables to a warm serving platter, discarding bay leaf; sprinkle with salt and pepper to taste. Garnish with parsley. Serve immediately.