Heat grill. Place 1 chicken breast half between 2 pieces of plastic wrap or waxed paper. working from center, gently pound chicken with flat side of meat mallet or rolling pin until about 1/4 inch thick; remove wrap. Repeat with remaining chicken breast halves.

In small bowl, combine mayonnaise and mustard. Cut four 18x12-inch pieces of heavy-duty foil. Place 1 chicken breast half on each; spread about 1 Tbsp. mayonnaise mixture on each breast half. Top each with 1 slice ham and 1 slice cheese. Wrap each packet using double-fold seals, allowing room for heat expansion.

When ready to grill, place packets, seam side up, on gas grill over medium heat, on charcoal grill 4 to 6 inches from medium coals, or on campfire grill over medium heat. cook 20 to 30 minutes or until chicken is fork tender and juices run clear, rearranging packets several times during cooking.

To judge the temperature of a fire, cautiously hold you hand, palm side down, over the fire at cooking height. Now, count the number of seconds you can hold that position and you will have an indicator of how hot the fire temperature is: