Heat your grill for indirect cooking and clean and oil the grate.
When ready to cook (I used charcoal), pat the chicken dry and pound to ½" thick.
This can be done ahead of time, and stored in ziplock bags until ready to cook if hiking in, just make sure to keep the chicken at a safe temperature on ice.
Season the chicken with salt and pepper and toss onto the grill.
Cover and cook, turning as needed for epic grill marks (and to make sure the heat is evenly distributed) until the chicken reads 160 degrees on an instant thermometer.
Wrap in foil and let rest another 5 degrees.
Meanwhile, toss the sliced peppers and onion in the oil, sprinkle with a little salt and toss onto a campfire safe pan or griddle, and cook until soft.
Slice the chicken and toss with the onion and pepper.
Slather the rolls in mayo, arrange the sliced apples, chicken, onions, peppers and cheese over the rolls and top with the other half of the roll.
Wrap tightly in foil and press onto the campfire until the bread is warmed through and the cheese is melty.
This can be done by tossing the foil directly onto the campfire if you have a safe means to retrieve your sandwiches, no need to make sacrifices to the camping gods. Or make a panini by pressing the wrapped sandwiches with the previously used griddle for pressed delicious grill marks.
Unwrap and serve hot.
Pound your chicken and slice your peppers and onions ahead of time and store in ziplock bags for easy campfire ready meal prep. Jazz up the recipe by included some bacon that you cooked at home and hiked in with.