Preheat the oven to 220ºC / Gas 7.
Remove the pork from the packaging and pat dry with kitchen roll. If the pork skin isn't scored, with a sharp knife score across the pork skin.
Place the sliced onion and sage leaves on the bottom of a roasting tin. Place the pork on top.
Rub oil and salt all over the crackling. Place the pork in the oven and cook for 30mins. Then reduce the heat to 180ºc/Gas 4 and cook for the remaining time according to the size of your roasting joint. (As a guide medium - 25mins per 450g, plus 20mins, if you prefer your pork well done, then 30mins per 450g, plus 20mins)
When cooked, remove from the oven and leave to rest for around 15mins, covered loosely in foil. When ready to carve, take off the crackling and slice.
