Preheat the oven to 190°C. Melt the butter in a frying pan, add the onions and cook on low heat until soft and golden. Put the onions into a bowl and add the celery, sage, chopped apple, breadcrumbs and sultanas or prunes. Season well, then mix in just enough beaten egg to bind the mixture together.
Make a deep horizontal cut the whole length of the thickest part of the loin, open out flat and season well with salt and pepper. Spread the stuffing evenly over the meat, then roll up and tie securely with fine kitchen string at 2–3 cm intervals along its entire length.
Put the meat in a roasting pan, cover with the reserved skin and cook in the centre of the oven, allowing 30 minutes per 500 g of meat. Remove the pork skin for the last 30 minutes of cooking time and baste the pork frequently with the pan juices until the meat is cooked and browned and the juices run clear when it is pierced with a skewer. Transfer the meat from the roasting pan to a heated serving plate and keep hot.
Skim off all but 1 tablespoon of the fat from the juices remaining in the roasting pan. Stir the flour into the juices and gradually stir in the stock and red wine. Bring to the boil, stirring continuously, then reduce the heat, stir in the redcurrant jelly, season to taste and cook for 5 minutes. Strain the gravy through a sieve into a heated gravy boat. Remove the string from the pork, garnish with sage and parsley and serve with the gravy, mashed potatoes and buttered red or green cabbage.