Season the pork on both sides. Place the onions and water in a large non-traditional christmas dish and place the pork on top.
Cover with a piece of baking paper then seal tightly with foil. Bake for 1 hour, then check to see if the pork is tender. If not, re-cover and continue to cook until tender. Uncover and spoon over the remaining glaze. Bake for a further 30 minutes until the top is golden and sticky. Transfer the pork to a plate and rest for 15 minutes before carving.
To serve: Tilt the roasting pan and spoon off excess fat. If the pan juices are thin, place over a medium heat and reduce down. Carve the pork into pieces or slices and arrange on a platter. Serve the onions and pan juices separately.
If you think the crackling is the best part of the pork belly and would like to serve it alongside, brush the reserved skin with olive oil and sea salt. Place in a baking dish and roast in a hot oven until golden and crisp. Remove the beef from the fridge half an hour before cooking. Put the celeriac rounds in a roasting dish with the thyme sprigs and lay the beef, fat-side up, on top.
Season well with salt and pepper. C and roast for a further 30 minutes. Cover the beef loosely with tinfoil and allow to rest for 15 minutes. Horseradish cream: Place everything apart from the reserved cornichons in a small food processor and whizz to combine. Serve immediately, garnished with the reserved cornichons, or store in the fridge for up to 6 hours before serving.