In large bowl, combine vinegar, sage and oil; mix well. Add lamb and onion; stir gently to coat. Cover; refrigerate 1 hour to marinate.
Thread lamb and onion pieces on 6-inch metal skewers; reserve marinade. Broil 6 inches from heat for 8 to 10 minutes or until lamb is no longer pink, brushing frequently with marinade. (Onion pieces will remain crisp.)