Preheat the oven to 250 degrees F. Generously butter a large baking sheet.
On a cutting board, turn an apple on its side and cut into very thin slices, about 1/8-inch thick. (I used a mandolin.) Using a small cookie cutter, cut the core out of each apple slice and discard.
Spread the apple slices out in one layer on the baking sheet. Sprinkle with cinnamon. Bake for 30 minutes, flip the apple pieces over, and bake for another 30-50 minutes until crisp. Depending on how evenly you sliced the apples, some may be done sooner than others. If so, remove the thinner ones when they are brown and crisp and allow the thicker ones additional time in the oven.
Paraphrased from the 100 Days of Real Food Cookbook.