Thaw fish, if frozen. In 10-inch skillet, cook onion and garlic in butter. Add fish, tomato, wine, parsley, salt and pepper. Cover; cook over low heat 10-12 minutes or until fish is done. Remove fish, keep warm. Blend milk and cornstarch; add to skillet. Cook and stir until thickened and bubbly. Cook 1-2 minutes more. Pour over fish.