Sprinkle ribs with salt and pepper. In 3 1/2- to 4-quart slow cooker, place ribs. Cover with onion slices. In small bowl, mix remaining ingredients; pour over ribs and onion.
Cover; cook on Low heat setting 8 to 10 hours.
Remove ribs from cooker; place on serving platter. Cover to keep warm.
Pour juices from cooker through strainer into 1-quart saucepan. Heat to boiling over medium-high heat; reduce heat to medium. Cook about 5 minutes or until sauce has slightly thickened. Serve sauce with ribs.