The day before, put the rub ingredients in a large freezer bag, add the pork belly, hold the top shut and shake to coat, then seal well. Leave in the fridge overnight.
To make the barbecue sauce, mix all the ingredients in a heavy-based pan and simmer gently, stirring, for 20 minutes until thick. Pass through a sieve into a bowl and leave to cool. Chill, covered, overnight.
Heat the oven to 160°C/fan140°C/gas 3. Scatter the onions in a large roasting tin. Remove the pork belly from the bag, put on top of the onions and pour over the cider. Cover the tin with foil and roast for 3-4 hours until tender and almost falling apart.
Light the barbecue half an hour before the end of the cooking time of the pork. Take the pork out of the oven, remove the foil, then peel off and discard the skin.
Brush the pork all over with some of the barbecue sauce. Put on the barbecue rack, above the coals, and cover with the barbecue lid. (Adjust the barbecue vents – closed vents will extinguish the fire and fully open vents might make it too hot.) Cook for 1 hour, turning often and brushing with more barbecue sauce. Once glazed and sticky, and the meat pulls away easily, it’s ready to eat. Shred the meat from the bones, then serve in buns with relish of your choice .