Preheat the oven to 220C/430F/Gas 7.
For the stuffing, mix all of the stuffing ingredients together and chill in the fridge for 30 minutes.
For the pork, lay 3-4 pieces of kitchen string across a chopping board and then place the pork belly on top, skin side down. Spread the stuffing mixture all over the pork belly, season with salt and freshly ground black pepper, then place the pork tenderloin across the middle of the belly. Roll up the pork belly, then tie up, using the string, at equal intervals.
Rub a generous amount of sea salt into the skin of the pork belly and roast for 20 minutes in the oven, or until the skin is beginning to crisp up and turn golden-brown. Turn the oven down to 180C/360F/Gas 4 and roast for a further three hours. Remove from the oven and allow to rest for 10-15 minutes before carving.
Meanwhile, for the gravy, heat the olive oil in a large pan until smoking, then add the pork bones. Cook for 6-8 minutes, or until the bones are browned all over. Remove the bones from the pan and set aside.
Return the pan to the heat and fry the onion, carrot and celery for 4-5 minutes, or until golden-brown. Add the leeks and thyme and cook for a further 4-5 minutes, stirring continuously.
Add the white wine to the pan and cook for 6-8 minutes, or until the liquid has reduced by half in volume. Return the pork bones to the pan and add the chicken stock, bring to the boil then reduce the heat and simmer for two hours.
Pass the liquid through a sieve into a clean pan and discard the solids. Simmer the liquid for a further 30 minutes, or until thickened to a gravy consistency. Keep warm.
For the ratatouille, place all of the vegetables, except the tomatoes, tomato purée and basil, into a bowl and toss together with the olive oil. Season with salt and freshly ground black pepper, then transfer to a roasting tray. Cook in the oven, with the pork, for 10-12 minutes, or until the vegetables are beginning to colour.
Stir the tomatoes, tomato purée and basil into the vegetables, then continue to cook in the oven for 20 minutes, or until all of the vegetables are tender. Remove from the oven and cover with aluminium foil to keep warm. Keep the oven at 180C/360F/Gas 4.
For the roast potatoes, pour the olive oil into a large roasting tin and place in the oven until smoking.
Add the parboiled potatoes, rosemary and garlic to the pan and, using a large spoon, carefully toss the potatoes to ensure they get a good coating of oil. Season with salt and freshly ground black pepper, then roast in the oven for 40 minutes, or until golden-brown and crisp. Set aside.
For the glazed apples, lay the apple slices on a baking tray and sprinkle over the brown sugar. Using a mini blow torch, caramelise the sugar until melted and golden-brown, then allow to cool slightly. (If you don't own a blow torch, place the sugared apple slices under a hot grill until the sugar melts and turns golden-brown.)
To serve, spoon the roast potatoes and ratatouille onto serving plates. Thickly slice the stuffed pork and divide among the serving plates, then spoon some of the glazed apples alongside.