Preheat the oven to 200°C. Weigh the pork and calculate the cooking time, allowing 25 minutes per 500 g plus 25 minutes.
Wipe the meat with paper towels. Using a very sharp knife, make diagonal scores in the skin across or vertically down 5–10 mm apart and about 3 mm deep.
Put the meat in a roasting pan and, to encourage the skin to ‘crackle’, rub it all over with the olive oil, then rub in the salt and scatter with rosemary.
Roast the pork in the centre of the oven for 30 minutes to start the skin crisping, then reduce the oven temperature to 180°C. Cook, basting every 30 minutes with the fat from the bottom of the pan, until the meat is well done.
About 1 hour before the pork is due to come out of the oven, put the potatoes into a large saucepan, cover with cold water, add the salt and bring to the boil.
Drain well, return to the pan and shake over high heat for 1–2 minutes until they are dry.
Heat the lard in a roasting pan on the shelf above the pork until it sizzles.
Add the potatoes, baste well and roast for 1 1/4 hours, until crisp and golden.
When the pork is cooked, remove it from the oven and leave it to rest in a warm place. Increase the oven temperature to 220°C and let the potatoes finish cooking.
About 30 minutes before the end of the pork’s calculated cooking time, put the cooking apples into a small saucepan with the water, allspice and sugar.
Cover and cook gently until soft and pulpy.
Remove from the heat and mash with a fork.
Peel and core the dessert apples, cut in half horizontally and brush each half all over with the lemon juice.
Melt the butter in a large frying pan, add the sage, then place the apple halves in the pan, cut sides up.
Top each half with the apple sauce and baste well with the sage butter. Cover the pan and cook gently, basting occasionally, for 10 minutes, or until the apples are just softened. Remove from the heat and keep warm.
At the end of the calculated cooking time, pierce the pork with a skewer at the thickest part.
The juices should run clear with no trace of pink. (If necessary, continue roasting until done.)
Lift the cooked pork from the roasting pan onto a large, heated serving plate, cover loosely with foil and allow to stand while making the gravy.
Skim off all but 2 tablespoons of the fat from the roasting pan, then stir the flour into the fat and juices remaining in the pan.
Cook over medium heat until well browned but not burned.
Gradually add the chicken stock and bring to the boil, stirring continuously and scraping any browned residue off the bottom of the pan.
Strain the gravy through a fine sieve into a saucepan, then blend in the redcurrant jelly and dry sherry, simmer for 5 minutes and season to taste.
Arrange the apple halves around the pork, garnish with the sprigs of sage and serve with the roast potatoes and gravy.
Buttered cabbage or green peas make a good accompaniment for this dish.