Pre-heat the oven to 240°C.
Score the pork skin into thin strips using a sharp knife.
Place pork in a tin skin-side up with onion halves wedged underneath.
Press crushed salt crystals evenly over the skin.
Roast on a high shelf for 25 minutes.
Reduce heat to 190°C and cook for a further 150 minutes.
Confirm the meat is cooked by checking that the juices run clear.
Remove from oven and let the meat rest for 30 minutes.
Spoon off excess fat from the tin juices while keeping the onion inside.
Whisk flour into the juices over low heat.
Gradually whisk in cider and stock until the gravy simmers and thickens.
Season to taste, discard onion, and serve gravy with carved pork and crackling.