Preheat the oven or air fryer to 200˚F. Wash the apples under cool running water and dry with a clean kitchen towel.
Slice the apples 1/8-inches thick using a sharp knife or mandoline. If you are using an air fryer, only prepare one apple at a time to fit the air basket.
Shake out the seeds and place the apple slices in a single layer on a work surface or baking sheet.
Sprinkle the apple slices evenly with cinnamon. Flip the slices over and sprinkle the other side.
Baked Apple Chips: Place the apple slices on two parchment-lined baking sheets, place into the preheated oven, and slow bake until crispy, about 2 hours. Flip the slices about half way through to cook evenly.
Air Fryer Apple Chips: Arrange the apple slices in a single layer in the air fryer basket. Air fry until crispy, 20 to 45 minutes. Flip part way through so they cook evenly.
Apples will continue to crisp up while cooling. To test for crispness, remove a couple apple chips and let them cool for a few minutes. Fold the cooled apple chip in half. It should snap when you bend them. If the apple chip is flexible and leathery, bake or air fry for 5 to 10 minutes more and test again.
Once your apples are done, turn off the heat and let them cool in the oven or air fryer. This will crisp up the apples further.
Remove from the oven and let the apple chips cool down completely. Store the apple chips in an airtight container for up to several weeks.