Preheat the oven to 220'C. Remove the pork from the fridge and discard all of the packaging well in advance of cooking - the pork will become crispier if it closer to room temperature before cooking.
Place the pork a sieve or colander and pour a kettleful of boiling water over it. This causes some of the rind to shrink back away from the fat. Pat the pork dry - the drier the better! Once dry, start scoring the skin to a depth of about 5mm. Start at one end of the joint and score on an angle all the way across. Keep the cuts about 1cm apart. Turn the joint around a repeat to create a criss cross pattern all over the skin.
Rub the salt into all the score marks. The salt draws moisture out of the joint which allows better crackling. This should be done just before cooking.
Slice the onions in half and place flat side down into a heavy based roasting tin. Place the pork on top of the onions, they will raise the joint off the tray allowing heat to circulate. Transfer to the oven and cook for 15 to 20 minutes before reducing the temperature to 160'C to finish cooking. A longer cook at low heat is better to retain the moisture in the meat. Check after an hour or so and cook for longer if necessary. The pork should be crisp and a deep golden brown colour. Once cooked, cut into slices and enjoy!
«You can also brine the joint overnight in salt water to help keep the joint moist when cooking.»