Chop cold bacon into pieces and cook them in the preheated pan over medium to medium-low heat until crispy. Drain off the bacon fat and set the bacon bits aside to cool. (Set aside some bacon bits to top off the chicken in the end.)
Dice apples and mix it with bacon in a bowl.
Slice cheese into about 1/8 inch slices.
Butterfly each chicken breast and be careful not to cut all the way through. Place opened butterflied chicken breasts on a large plastic cutting board and cover them with some plastic wrap. (This will precent any chicken juices from splattering while you tenderize it.)
Gently pound each chicken breast with the flat wise of the meat tenderizer until thinned. Be careful not to hit it too hard so you don't have any holes in the chicken. Also, don't make it too thin or it will rip while you are trying to roll it with stuffing.
Discard the plastic wrap and salt and pepper each chicken breast to taste.
Place two slices of cheese in the center of chicken breast. Add some apple/bacon mixture on top of cheese. You'll have to judge how much stuffing to add by the size of the chicken. Each breast varies in size and you should be able to bring edges together with just a little overlap. Once you closed the chicken, place it in a greased baking dish or a cast iron skillet, seam down.
Repeat with remaining chicken. Add a little more salt and pepper on top.
Bake chicken for 30-35 minutes.
Spread some grated cheddar cheese on top of each chicken pocket and sprinkle some diced bacon over it.
Bake for just a few more minutes to melt the cheese.