Pre-heat the oven to 240C/475F/Gas 9.
While the oven is preheating, score the skin of the pork. It will be scored already, but it's always best to add a few more lines. To do this you can use the point of a very sharp paring knife, or Stanley knife, or you can now even buy a special scalpel from a good quality kitchen shop! What you need to do is score the skin all over into thin strips, bringing the blade of the knife about halfway through the fat beneath the skin.
Now place the pork in a tin, skin-side up, halve the onion and wedge the two pieces in slightly underneath the meat. Now take about 1 tbsp of crushed salt crystals and sprinkle it evenly over the skin, pressing it in as much as you can.
Place the pork on a high shelf in the oven and roast it for 25 minutes. Turn the heat down to 190C/375F/Gas 5, and calculate the total cooking time allowing 35 minutes to the pound, then deduct the initial 25 minutes cooking time. In this case it would be a further 2½ hours.
There's no need to baste pork as there is enough fat to keep the meat moist. The way to tell if the meat is cooked is to insert a skewer in the thickest part and the juices that run out should be absolutely clear without any trace of pinkness.
When the pork is cooked remove it from the oven and give it at least 30 minutes resting time before carving. While that is happening, tilt the tin and spoon all the fat off, leaving only the juces. The onion will probably be black and charred, which gives the gravy a lovely rich colour. Leave the onion in, then place the roasting tin over direct heat, turned to low, sprinkle in the flour and quickly work it into the juices with a wooden spoon.
Now turn the heat up to medium and gradually add the cider and the stock, this time using a balloon whisk until it comes up to simmering point and you have a smooth rich gravy. Taste and season with salt and pepper, then discard the onion and pour the gravy into a warmed serving jug. Serve the pork carved in slices, giving everyone some crackling.