Via: Yuki Sugiura
Using a very sharp knife, score the pork at 1cm intervals. Scatter some salt over the skin and leave for 1 hour. Preheat the oven to 220°C/gas mark 7.
Pat the meat dry and shake off any excess salt. Pile the onions and rosemary in a roasting pan and lay the pork belly on top. Try not to leave any stray pieces of onion poking out from under the pork. Roast in the oven for 20 minutes, then turn the oven down to 160°C/gas mark 3, and roast for a further 2½ hours.
Meanwhile, make the salsa verde by thoroughly blending or finely chopping all the ingredients apart from the olive oil. Slowly stir or blend in the oil and season with salt and pepper if necessary. Cover and chill.
Remove the pork and transfer to a carving board to rest. If the crackling isn't as crisp as you'd like, put the pork in a different roasting pan and return to the oven at 220°C/gas mark 7 until crisp. Tip away the excess fat from the original roasting pan, then put the pan over a high heat. Add the cider and simmer for a minute or so, scraping up all the pan juices, then add the stock and simmer for 10 minutes. Taste for seasoning and add a little salt and pepper if necessary. If you've been crisping up the crackling, rest the pork belly for 5-10 minutes before serving.
Carve the pork and crackling into nice thick chunks and serve with a spoonful of the gravy and a good dollop of salsa verde.