Preheat oven to 240°C. Score diamonds into the pork skin with a very sharp knife. Place the pork skin-side down and rub the flesh with about 1 tbsp of oil, a healthy pinch of salt and the Crunchy Pork Crackling Rub. Roll the pork to form a cylinder and tie at about 3cm intervals with kitchen string to secure.
Place the pork into a roasting tray and drizzle with another tbsp of oil and another pinch of salt, rubbing it into the skin. Place the pork in the hot oven for about 20 min or until the crackling starts to form, then reduce the heat down to 150°C. Scatter the apple, onions and sage around the pork, pour in the cider, cover tightly with foil and roast for a further 1–1 ½ hours.
Increase oven to 250°C and uncover the pork. Roast for a further 15 min at high heat until the crackling is crisp. Remove from the oven and allow to rest for about 10 minutes before slicing and serving.