Combine the first 8 ingredients in an extra-large zip-top plastic bag. Cut onion into 6 wedges, leaving root end intact. Add onion and next 5 ingredients (onion through yellow bell pepper) to bag. Seal and marinate in refrigerator 2 hours, turning bag occasionally.
Remove vegetables from bag. Place vegetables on a grill rack coated with cooking spray; grill 6 minutes on each side or until tender.