Pre-heat the oven to 240°C/475°F/gas mark 9. Using the point of a sharp knife, score the skin of the pork. Cut vertical lines a finger width apart about halfway through the fat beneath the skin. This is to allow the heat to penetrate the fat and bubble up through the cuts to baste the meat as it is cooking. Dry the skin and rub some salt into the cuts. For good crackling the skin must be dry.
Place the onion in the roasting dish and the pork on top of the onion wedges, skin side up. Put the tin in the oven on a high shelf for 30 minutes, then turn the heat down to 180°C/360°F/gas mark 4 and cook according to the size of your joint (see below)
Cut the potatoes into egg size. Bring a pan of lightly salted water to the boil and simmer for 8 - 10 minutes. Drain through a colander and leave them to cool. Give the colander a shake to roughen the edges of the potatoes, this helps the fat to stick and makes them more crispy.
To make the apple sauce
Peel & chop the apples and put into a pan with 4 tablespoons of cold water.
Bring to a boil, then reduce the heat to medium, cover with a lid and cook for 6 - 8 minutes until the apples are soft.
Take the apples off the heat and beat them with a wooden spoon until they are smooth. Stir in the butter and sugar. If the sauce is too thin, put it back on the heat and stir gently until it thickens. Transfer to a serving bowl.
50 minutes before eating
Put the potatoes in a separate roasting tin. Drizzle over a few tablespoons of vegetable oil. Place the tray in the oven with the meat.
30 minutes before eating
Check the pork is cooked (see not below). Remove it from the oven, cover loosely with foil and leave it to rest for 30 minutes. Turn the heat up to 220°C/425°F/gas mark 7 to help crisp off the potatoes. Chop any vegetables such as carrots ready for boiling.
20 minutes before eating
Fry the leeks in the butter in a pan until softened. Stir in the flour and the milk and bring to a simmer. Stir the cheese into the sauce until melted. Turn the heat off. Reheat a couple of minutes before serving.
15 minutes before eating
Place carrots in a pan of cold water and bring to a boil. Simmer for 10 minutes until soft. Drain and keep to one side.
Place the meat on a carving dish. Place the roasting tin over a medium heat and heat the meat juices then sprinkle in the flour and stir with a wooden spoon until you have a paste. Gradually add the stock stirring all the time until it is simmering and the gravy is smooth. Taste and season with salt and pepper if necessary. Strain it into a warmed jug.
5 minutes before eating
Steam the cabbage or broccoli for 5 minutes until tender
Carve the meat. Remove the roasted potatoes from the oven and serve together with the vegetables, cheesy leeks, gravy, apple sauce and crackling!