Preheat the oven to 180°C.
Use a sharp knife to cut 4 or 5 deep holes lengthwise into the joint, then push the rosemary garlic into them.
If the skin is not already scored make half a dozen long cuts halfway through it. Rub the oil all over the joint, particularly into the skin.
Place the slices of apple and onion across the bottom of a small roasting tray.
Put the joint, skin upwards, on top of the slices then sprinkle over the seasoning & herbs.
Cook in the oven for about 1¾ hours. About halfway through lift off the joint, turn the slices over and replace the pork with its rind sideways.
When the cooking time is up, remove the crackling from the joint and, if necessary, pop it back into the oven for 5-10 minutes to crispen further.
Carve and serve with the slices of apple & onion and vegetables of your choice - sprouts and roasted sweet potatoes go particularly well.