Combine first 8 ingredients in a medium bowl, stirring well with a whisk. Add chicken, tossing to coat; cover and marinate in refrigerator 3 hours.
Cut onion wedges in half crosswise. Remove chicken from yogurt mixture, discarding yogurt mixture. Thread chicken, onion chunks, and bell pepper pieces evenly on 4 (12-inch) skewers, alternating chicken and vegetables.
Place skewers on grill rack coated with cooking spray. Grill 15 minutes or until done, turning twice.