1. Trim fat from lamb; cut lamb into 1 1/4-inch cubes. Place lamb in a shallow dish. Combine yogurt, soy sauce, and curry powder; add to lamb, and stir to coat. Cover and marinate in refrigerator at least 2 hours.
2. Cook onions in boiling water to cover in a medium saucepan 10 minutes. Drain and cut each onion into 4 wedges.
3. Prepare grill.
4. Remove lamb from marinade; reserve marinade. Thread lamb, onion, bell pepper, and mushrooms alternately onto 8 (10-inch) skewers; brush with reserved marinade.
5. Coat a grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place kabobs on rack; grill, covered, 10 minutes or to desired degree of doneness, turning kabobs once. Serve immediately.