These thin slices will need a couple of hours in the oven to dry out and firm up, but it's mostly hands-off—you'll just need to flip them halfway through.
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.