Bring the pork to room temperature and score the surface of the pork with fine lines.
Sprinkle the surface with table salt and leave for 30 minutes.
Heat the oven to 160FAN/180C/350F.
Wash off the salt and pat dry then put the pork on a rack over a roasting pan.
Rub the meat with vegetable oil and sprinkle with the sea salt.
Cover with foil and roast for an hour and a half.
Slice the onions and cut the apples into wedges and place at the bottom of the oven tin.
Return to the oven for another hour.
Remove the foil and increase the oven to 180FAN/200C/400F for 30 minutes to crisp up the crackling.
Place it on a carving tray and cut off the crackling and put it on a separate plate.
Cover the pork loosely with foil and leave to rest for at least 15 minutes.
Drain the fat from the tray, reserving the juices, onions and apples.
Slice the pork and serve with chunks of crackling and the apples and onions on the side.