Position oven racks in the upper and lower third of oven.
Preheat oven to 225 ℉.
Line 2 baking sheets with parchment paper. Set aside.
Wash and dry the apples.
Core apples (optional).
Slice apples as thin as possible (about 1/8 inch) with a very sharp knife or a mandolin (leave skin on).
Place apples on the baking sheet in a single layer and not touching each other.
Sprinkle cinnamon evenly over apple slices.
Bake for approximately 1 hour and then turn each slice over, and continue to bake for at least another hour so. The longer they stay, they crispier they get.
Best served the day they are made. Can be stored in an airtight container for a few days.