Wash and thoroughly dry the pork, then cut into 4 thick slices. Rub with plenty of seasoning. Heat a large casserole dish on a medium flame and add the pork belly, searing for 3 minutes on each side. You might need to do this in two batches. Set the pork aside.
Preheat the oven to 180°C/fan 160°C/gas mark 4. Add the onion, carrot, celery, garlic and fennel seeds to the casserole dish and cook for 10 minutes until soft and fragrant. Add the lentils and stir through, cooking for a further 1 minute. Pour in enough stock to just cover the lentils, then lay the pork on top. Cover and transfer to the oven for 45 minutes.
Combine the mustard and honey and turn the oven up to 200°C/fan 180°C/gas mark 6. Remove the lid of the casserole dish and baste the pork with the honey and mustard marinade. Place back in the oven, uncovered, and roast for a further 40 minutes, basting often until the pork is dark and sticky. Add a little more water to the lentils if they seem dry. Serve.