1Heat gas or charcoal grill. Cut 4 (18x12-inch) sheets of heavy-duty foil. Place 1 chicken breast, 1 bell pepper wedge, 2 tomato halves and 2 onion wedges on center of each sheet. Pour 2 tablespoons dressing over chicken and vegetable mixture on each packet.
2For each packet, bring up 2 sides of foil over chicken and vegetables so edges meet. Seal edges, making tight 1/2-inch fold; fold again, allowing space for heat circulation and expansion. Fold other sides to seal.
3Place packets on grill over medium heat. Cover grill; cook 18 to 22 minutes, rotating packets 1/2 turn after 10 minutes, until juice of chicken is clear when center of thickest part is cut (170°F). Place packets on plates. Cut large X across tops of packets; carefully fold back foil.