Trim fat from lamb; cut lamb into 1 1/4-inch cubes. Place in a shallow dish. Combine lime juice and next 7 ingredients; pour over lamb. Cover and marinate in refrigerator at least 4 hours.
Cook 2 onions in boiling water to cover in a saucepan 10 minutes. Drain; cut each onion into 4 wedges. Remove lamb from marinade, reserving marinade. Place marinade in a saucepan, and bring to a boil. Remove from heat. Thread lamb, onion, green bell pepper, and mushrooms onto 8 (10-inch) skewers.
Place kebabs on grill rack coated with cooking spray; grill, covered, 13 to 15 minutes or until lamb is done, turning and basting frequently with reserved marinade.