Heat 1 tbsp oil in a large frying pan over a medium heat, add the pickled onions and fry for 3-4 mins until they get nicely caramelised. Add the cayenne and thyme leaves, then tip the roasties into the pan along with another 1 tbsp oil. Use the back of a wooden spoon to squash each one slightly. Leave alone for 3-4 mins to fry and get extra crisp on one side, then give them a good toss.
Add the remaining oil, toss and fry again for extra crispiness. Don’t worry if they fall apart a little, the scuffed bits will get extra crisp. Season to taste and serve warm with your Christmas leftovers.