Combine first 6 ingredients; set aside.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place chicken and remaining 4 ingredients on rack; grill, covered, 8 minutes on each side, basting often with juice mixture. Remove onion from grill, and keep warm. Grill chicken and remaining vegetables 2 to 3 additional minutes or until chicken is done and vegetables are tender. Transfer chicken and vegetables to a serving platter.