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The day before, use a very sharp knife to cut the skin away from the pork, being careful to leave the fat attached to the meat (or ask your butcher to do this). If not done already, score the skin in a criss-cross pattern, then pat dry with kitchen paper. Season the skin liberally with salt and put on a plate in the fridge, uncovered, to dry overnight.
Meanwhile, heat oven to 180C/160C fan/gas 4. Put the onions, bay, thyme, garlic and fennel seeds in a large flameproof casserole dish. Season the meat liberally, then nestle it into the tin. Pour over the cider, pop on the lid and cook in the oven for 2½ hrs. Remove from the oven and, once cooled slightly, place the pork on a tray or in a container. Pour the liquid into a jug, cover and put everything in the fridge to chill overnight.
The next day, lift off any fat that has hardened on the surface of the braising liquid, then spoon it back into the dish with the vegetables. Add the vinegar, apples and sugar, and simmer for 1 hr. Leave to cool slightly, then remove the herbs and garlic. Blitz in a blender and taste for seasoning – you want a balance of sweet and sharp, so adjust the sugar and vinegar if you need to, then pour through a sieve to make a thick, pureéd ketchup.
To reheat the pork and make the crackling, fire up the barbecue or heat the oven to its highest setting. If cooking outside, lay the crackling on the grates and cook, turning occasionally, until it is puffed up and golden, while the pork is on the other side of the barbecue, heating through and nicely browning. Leave to rest for 10 mins before bringing to the table and carving. If cooking in the oven, lay the crackling on a wire rack over a baking tray on the top shelf and cook until crisp, while the pork reheats and browns underneath. If necessary, turn on the grill while the pork rests to really crisp up the crackling.