1: Preheat the oven to 210°F (100°C). Rinse and dry 1 apple. Remove the core with an apple-corer (as in this photo). You can peel the apple, if you prefer, but this is not essential.
2: Slice the apple(s) very thinly. The best tool for this is a mandolin.
3: Arrange the apple slices, without overlapping, on a sheet of cooking parchment and slide this onto a baking sheet.
4: You can sprinkle a little caster sugar over the apples if you like, but this is not essential.
5: Bake in the oven for about 2 hours. To check if the crisps are ready, take one off the baking sheet and leaving it to cool. It should be brittle and crisp, not soft and bendy.
6: Unstick the apple slices from the paper (which you can reuse) while they are still hot and leave to cool on a wire rack.
Remarks
You will get the best results with well-flavoured, slightly sharp apples, such as reinettes or Cox's orange pippin.