1. Preheat the oven to 180°C, gas mark 4. Untie the pork and lay it upside down on a board. Using a sharp knife cut down the length of the meat to open it to create a fairly flat surface.
2. To make the stuffing, remove the skin from the sausages and place the meat in a bowl and add the breadcrumbs, grated apple, sage, pine nuts and season. Mix together and then shape to fit down the centre of the opened-up pork loin. Fold the meat and skin back over the stuffing and press together to resemble the original shape. Lay seam side down and tie with string at intervals to hold it together.
3. Arrange the rounds of onion in the bottom of a roasting tin. Scatter over the remaining sage leaves and sit the pork loin on top, seam side down.
4. Rub the joint all over with salt and pepper and pour about 300ml of the cider into the bottom of the tin. Place in the oven and roast for 20 minutes per 450g plus an extra 20 minutes, about 1½ hours in all. Make sure the liquid in the bottom of the tin doesn’t dry out – top up with more cider and water when needed.
5. When cooked through with no pink meat, remove the pork from the roasting tin and transfer to a carving board. Cover with foil and leave to rest while you prepare the gravy.
6. Place the roasting tin (including the onions) on the hob and bring to a simmer. Sprinkle over the plain flour and beat in with a wooden spoon then gradually pour in the stock and any remaining cider and simmer until you have a thickened gravy. Strain though a sieve into a jug making sure you press through all the juices from the onions. Serve with the slices of pork and stuffing and a Swiss chard gratin.