Place the joint, flesh side down on a chopping board. Take a sharp knife to cut through the skin and fat 4cm from the rib end, down to the bone. Use the knife to pierce the flesh between the bones.
Turn the meat over so that the bones face upwards. Widen the cut holes and then cut along the lengths of bone to the end. Cut around the bones.
Peel and slice an onion and use it to line a roasting dish where the pork will sit.
Turn the joint over again and scrape the bones until they are clean.
Score the pork rind for the crackling with a sharp knife.
Rub the skin with oil and sprinkle on the salt. Push it in with your fingers then place the joint on the onion trivet.
Preheat the oven to 220C/200FAN/425F/Gas 7 and roast the meat for 20 minutes.
Turn the heat down to 200C/180FAN/400F/Gas 6 for 30 minutes per 500g or 1 lb. The juices should run clear when the meat reaches an internal temperature of 70C or 160F.
Remove the pork and place on a roasting tray, loosely covered with foil for a minimum of 20 minutes.
Pour off most of the fat from the roasting pan then place it over a medium heat and add the chicken stock. Keep stirring until all the grungy stuck on bits have loosened.
Tip the mixture into a liquidiser and blend the onions and cornflour in directly and then return to the roasting pan to thicken and heat through.
Slice the pork with the crackling and serve with the gravy poured over.