1) Preheat the oven to 230 degrees C/Gas 8. Spray a baking tray slightly with oil and set aside.
2) Place potato crisps in the bowl of a food processor and process into crumbs, about 20 seconds. Transfer to a shallow bowl, add cayenne, and set aside.
3) In another bowl, combine buttermilk, 2 tbsp of flour, salt and pepper and set aside.
4) Slice onions into 1.3-cm circles and separate into rings, keeping only large, whole rings (reserve rest of onions for other uses). You should come out with about 12 to 14 rings.
5) Place the remaining flour in a sealable plastic bag, then add onions, and shake to coat. Dip onions 1 at a time into the buttermilk mixture, then dip into potato crisp crumbs and place on baking tray.
6) Spray rapeseed oil evenly over rings and bake for 20 minutes, or until coating is crisp.
7) Season with salt, to taste, and serve immediately.