Preheat oven to 225ºF. Lay out the apple slices on two parchment-lined baking sheets.
Mix together the sugar and cinnamon in a small bowl. Then sprinkle the mixture over the tops of the apples. If using maple syrup, simply brush the tops of the apples.
Place both baking sheets in the prepared oven and bake for 1 1/2 hours. Flip apples over and continue baking for 1 more hour, until the apples are crispy. Let cool and store in an airtight container for up to 1 week. Enjoy!