Coat slow cooker with cooking spray, arrange the ribs, sliced onion and chopped garlic in the cooker. Pour pineapple juice over the ribs.
cover and cook for 6 hours.
drain the cooking juices. Remove onions from slow cooker, and discard them. In a small bowl, stir together the barbecue sauce and the plum jam. Pour over the ribs and spread to that the ribs are evenly covered with sauce.