Place onion in a 3-1/2- to 6-quart slow cooker. Place ribs on top of onion in cooker. In a medium bowl stir together remaining ingredients. Reserve 1 cup juice cocktail mixture for sauce; cover and refrigerate. Pour remaining juice cocktail mixture over ribs.
Cover and cook on low-heat setting for 10 to 12 hours or on high-heat setting for 5 to 6 hours.
For sauce, in a small saucepan heat reserved juice cocktail mixture to boiling; reduce heat. Simmer, uncovered, for 10 minutes. Transfer ribs to serving platter; discard cooking liquid. Serve sauce with ribs. Makes 4 to 6 servings.