Prepare the pickled onion an hour ahead. Finley slice the onion and mix all the ingredients together in a bowl. You can double the batch and keep the onion you don't use in the fridge to go with  all your other meals, it's delicious!

Put a frying pan on a medium high heat, add the pork belly (no need for oil, there is enough fat in the belly), and cook for 7-8 mins to render all the fat from the belly and it starts to crisp up.