1Heat gas or charcoal grill. Cut 4 (18x12-inch) sheets of heavy-duty foil. Mix cheese, lemon peel and oregano. Place 1 chicken breast, 3 tomato slices, 1/4 cup onion and 5 olives on center of each sheet. Spoon 1/4 of the cheese mixture over chicken and vegetables on each sheet.
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 20 to 25 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 top of each packet; carefully fold back foil.