Make some deep incisions in the pork, close to the bone. Mix together the garlic, chopped rosemary and 1 teaspoon salt. Rub into the cuts, spreading any extra over the meat. Set aside for 20 minutes. Meanwhile, preheat the oven to 220C/gas 7/fan 200C.
Scatter the onion and a few sprigs of rosemary into a roasting tin. Put the pork on top, skinned side up, cover tightly with foil and roast with the rind for 30 minutes.
Turn down the heat to 190C/gas 5/fan 170C and cook the rind for a further 30 minutes and the roast for a further hour.
Remove the foil and roast uncovered for 20 minutes.
Meanwhile, prepare the apples. In a pan melt the butter and cook the onion gently until soft but not browned, about 10 minutes. Off the heat, add the lemon zest, breadcrumbs and rosemary. Season well. Core the apples. Cut a line through the skin all round the waist of each. Fill each apple with onion stuffing. Mix the vinegar and redcurrant jelly for the glaze.
Remove the pork from the oven. Turn the heat to 200C/gas 6/fan 180C. Stand the apples around the pork, brush the pork with a little glaze and drizzle a little over the apples. Return to the oven and roast with the rind also in the oven, uncovered, for a further 20-25 minutes. Baste the pork twice with the juices and brush once with any remaining glaze. Test the pork by piercing in the centre with a skewer. The juices should run clear, not pink. If in doubt, remove the apples, keep warm, cover the pork with foil and cook for 10-15 minutes more.
Put the pork and apples on a warmed plate, cover with foil and leave in a warm place for 15 minutes. Spoon off the excess fat from the tin and put the tin on the stove over a medium-high heat. Pour in the stock. Boil vigorously for 5-6 minutes. Taste and season, adding a little jelly and/or cider vinegar to taste. Strain into a gravy boat. Serve the pork and apples with the crackling, garnished with rosemary.