Cut each slab of ribs into 2- to 3-bone sections and place in a large bowl; toss with onion and apple juice. Layer ribs in a baking dish and pour steak sauce evenly over top. Cover and refrigerate at least 12 hours, turning once or twice.
Preheat oven to 350°F. Cover baking dish tightly with aluminum foil. Bake ribs until very tender, about 1 1/2 hours; halfway through baking, remove cover and rotate ribs. Cool to room temperature in baking liquid.
Preheat grill to medium-high. Coat ribs with sauce and grill until well-browned, turning as needed to brown evenly, about 5 minutes. Serve hot.
Makes 6 appetizers or 4 main-course servings.