Slice apples as thinly as you can. Grate the mozzarella cheese.
Spray a skillet or griddle with cooking spray. Lay one piece of bread in the hot skillet and top with thinly sliced apples, and 1/4 cup cheese. Top with second slice of bread.
Grill both sides of the bread and allow cheese to melt. Repeat until you have made four sandwiches.