Pre-heat your oven or barbecue to 120C. Season the pork shoulder well with salt and pepper and cook until internal temperature is at least 88-92C – about 6 hours.
Cut the celeriac in half and place on a baking tray. Mix the salt with a little water until it has the texture of wet snow and pack around the celeriac. Cook with the pork for approximately 3 hours.
While the pork and celeriac are cooking take one stick of rhubarb and cut into 6 pieces. Chop up the rest and add with the sugar and sherry vinegar to a pan and bring to a gentle simmer. When the rhubarb is beginning to lose its texture and is cooked through blitz to a puree in a food processor. Roast the 6 pieces with a sprinkling of sugar in a hot oven (200C) for 10 minutes.
When the pork is at temperature you can turn up the temp to get the skin to crackle but keep an eye on it so that it doesn’t burn. Rest for 30 minutes then you should be able to pull it apart. Crack the salt casing on the celeriac and remove the celeriac and slice. Serve with the rhubarb.