Heat oven to 225 degrees. Arrange apple slices on two parchment-lined baking sheets and bake for 1 1/2 hours.
Flip, then continue baking until crisp, about 1 hour more. Remove and let cool completely. Chips keep, stored in an airtight container, 1 week.
Sweeter apples, such as Gala, Honeycrisp, and Pink Lady, really shine in this snack.