Rinse and pat dry fish fillets. Place fish in 10-inch by 8-inch oven-proof glass or stainless steel baking dish. Combine vinegar, lemon juice, dried mustard powder, and chopped onion in a small mixing bowl. Pour over fillets. Cover dish and marinate fish in refrigerator for approximately 2 hours. Preheat broiler. Remove cover from dish and place dish in broiler for approximately 4 to 6 minutes, or until fish is white and flaky throughout. Garnish with parsley and onion slices. Yield: 4 servings Serving size: 4 ounces Calories per serving: 140 Per-serving composition: 4 very lean protein (courtesy of Lou Aronne, M.D. and Kathy Isoldi, R.D. from Weigh Less, Live Longer) Copyright, 1997, TV FOOD NETWORK, G.P., All Rights Reserved Recipe by: TVFN: RECIPE FOR HEALTH SHOW #RHE167 Posted to MC-Recipe Digest V1 #640 by 4paws@netrax.net (Shermeyer-Gail) on Jun 09, 1997