- Preheat the oven to 90˚C / gas ¼.
- Line 2-3 large baking trays with greaseproof paper.
- Slice the apples and bananas very finely with a mandolin or sharp knife and lay the slices on the lined baking trays in a single layer.
- Put the trays into the preheated oven and bake for 1½-2 hours. The apples will be crisp before the bananas , so check them after 1½ hours. The bananas can take up to half an hour longer in the oven, depending on their thickness.
- Leave the fruit crisps to cool on the tray, then transfer to an airtight container and eat within 1 week.