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.