Put the onions and celery sticks into a large, deep baking tray, then sit the pork on top, skin-side up, and roast for 45 minutes, until starting to crisp.
Turn down the heat to 160C/fan 140C/gas 3, pour the chicken stock and cider around the meat (being careful not to touch the skin), and roast for a further 3 hours. The meat should be very tender and the crackling crisp – if it’s not quite done, turn up the oven to 200C/fan 180C/gas 6 and roast for a further 15-20 minutes or until really crisp.
Move the pork onto a serving or carving plate with the onions, and loosely cover with foil. Pour the juices into a jug and allow them to settle. Skim off the fat, then pour the cooking juices into a pan and top up with just-boiled water if it tastes a little strong. Whisk in the wholegrain mustard.
Carve the meat and serve with the onions, celery and gravy.
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