Preparation -
Combine all the marinade ingredients in a shallow oblong baking dish and mix well; set aside. Place 1 piece of chicken between 2 pieces of waxed paper. Gently pound the piece of chicken with the flat side of a meat mallet until it's about 1/4" thick. Repeat with remaining pieces. Place the chicken breasts in the marinade, turning to coat on all sides. Cover and refrigerate for 4 to 6 hours, turning occasionally
Prepare a charcoal fire or a gas grill. Remove the chicken from the marinade. Discard the marinade. Place the chicken on the prepared grill, about 8" above medium hot coals. Grill for 8 to 10 minutes or until the chicken is fork tender and juices run clear, turning one time. Move the grilled chicken to the edge of the grill, away from the hot coals. Meanwhile, spread butter or margarine on one side of each bread slice. Place the bread slices on the grill, buttered side down. Grill until lightly browned; set aside. Place 1 tomato slice, 2 avocado slices and 1 slice cheese on top of each chicken breast. Cook for about 3 more minutes or until the cheese begins to melt. Place each piece of chicken, with the toppings between 2 grilled bread slices. Makes 4 sandwiches.