These days, pork youfind in the supermarket has often been treated in processing with water and glucose to make it moister and heavier. That's why when you cook it, it shrivels down to two thirds its uncooked size. Even when your joint of pork is free from these additives, it can be difficult to treat it in a way that results in roast pork with a popcorn-crisp, crackling skin. When you do manage it, puffed, salty crackling is a delectable thing of wonder. The technique has a lot to do with using varied cooking temperatures, and absolutely everything to do with making sure the skin is prepared properly before it even gets anywhere near the oven.

Choose a rare breed joint that has plenty of fat under the skin. Try a 2kg rolled loin: enough to serve six people with plenty for sandwiches later. Although convenient, rolled joints are also hard to make crackle, especially where the skin meets the roasting tin. The skin of your pork must be dried thoroughly with paper kitchen towels, and scored well. Even if your butcher has already scored it, you will probably benefit from making sure the scoring is fine and regular, so you will want to add your own cuts to the skin. Use a craft knife on the cold skin of the meat (this is easiest when the skin and fat are cold and firm), scoring it in lines about half a centimetre apart. When the joint cooks, the fat will melt and bubble through those lines, crisping the skin it touches. Rub salt into the skin, as if the pork were somebody you are particularly fond of who is demanding a lovely exfoliating massage.

If you've prepared the joint the day before, rub some more salt into the skin on the day you cook it, making sure it gets through the cracks where you scored it and into the fat. Put a bed of onions at the bottom of a metal roasting dish and rest the pork on top of it. Heat up a large knob of good, rare breed pork dripping (use goose fat in preference to one of those white blocks of lard) over a high flame in a small saucepan and pour the searing hot fat over the skin, then put the roasting tin in the oven at a very hot 220°C. After quarter of an hour, lower the heat to 180°C and cook the joint for two hours, basting every 20 minutes. Finally, turn the heat back up again for a final quarter of an hour – this should cause the skin to puff up and crackle deliciously. Enjoy!