Preheat the oven to 160˚C(325˚F), Gas Mark 3. Heat 1 tablespoon of oil in a heavy-based pan over a medium heat. Add the pork belly and brown all over, turning regularly with tongs. Transfer to a casserole dish. Add the carrots and onion to the pan and cook for a further 5 minutes, until golden brown, stirring regularly to ensure they cook evenly. Tip the vegetables over the seared pork belly and stir in the beef stock, red wine, apple juice, garlic and herbs. Cover tightly with a lid or foil and bake for 3 hours, until the pork belly is meltingly tender. Remove from the oven and leave to sit for 1 hour in the braising juices, then remove, cut the string and wrap twice in foil. Reserve the braising juices to use in the sauce. This is all best done 24 hours in advance. To reheat the pork belly, put a large frying pan on a medium heat with the remaining tablespoon of oil and the butter. Cut the pork into 12 even-sized slices and arrange in a single layer in the heated frying pan – you may have to do this in batches. Cook for 2-3 minutes on each side, until just golden. Transfer to a place and keep warm. Add the honey, soy sauce and star anise to the pan and allow to bubble down for 1-2 minutes, until syrupy. Return the pieces of pork belly to the pan and cook for another 3-4 minutes basting regularly, until sticky and caramelised.
To serve, place 2 slices of the caramelised pork belly on each warmed serving plate and add some wilted spinach. Spoon the honey and ginger sauce alongside the pork belly and serve the mashed potatoes in a separate warmed dish.