1. In 6-quart Dutch oven or saucepot, heat first 5 ingredients to boiling. Reduce heat to low; cover and simmer 1 hour, occasionally turning riblets.
2. Remove riblets from Dutch oven; cool until easy to handle, about 15 minutes.
3. Meanwhile, preheat broiler if manufacturer directs. In 10" by 6" baking dish, combine mayonnaise, dry mustard, milk, salt, onion, and garlic. On sheet of waxed paper, place bread crumbs. Dip each riblet into mayonnaise mixture and then coat with bread crumbs.
4. Place riblets on rack in broiling pan. Drizzle half of butter or margarine over riblets. Broil riblets 15 minutes or until browned. Turn riblets and drizzle with remaining butter or margarine. Arrange tomato wedges on same broiling rack. Broil 15 minutes longer or until riblets are evenly browned and tomato is heated through. Garnish riblets with broiled tomato wedges.