Score the fat of the pork belly, roll up tightly and secure with several pieces of string. Rub the skin with the olive oil and season with salt and freshly ground black pepper.
Place the onion and thyme in a flameproof baking tray and season with salt and freshly ground black pepper. Place the pork on top and cook in the oven for 30 minutes, or until the skin is crispy and golden-brown.
Reduce the oven temperature to 170C/325F/Gas 3. Add 200ml/7fl oz water and the white wine and cook for a further 2 hours.
Remove the tray from the oven, remove the pork and set aside to rest for 15 minutes. Turn the oven up to 200C/400F/Gas 6.
Place the baking tray onto the heat, add the stock and bring to the boil, using a wooden spoon to scrape up any browned bits. Season with salt and freshly ground black pepper and strain the gravy into a jug.
Meanwhile for the brioche and leek bake, heat 50g/2oz of the butter in a frying pan and fry the leeks and thyme for 2-3 minutes, or until softened.
Add the wine and cook until the volume of the liquid has reduced by two-thirds, add the cream and reduce by half again. Season to taste with salt and freshly ground black pepper and pour into an ovenproof dish.
Heat a separate frying pan until hot. Add the remaining butter and the brioche, and cook until golden-brown on all sides. Season to taste with salt and freshly ground black pepper and scatter over the top of the leeks.
Sprinkle the cheese on top of the dish and cook in the oven for 5-10 minutes, or until golden-brown and bubbling.
To serve, carve the pork into thick slices. Spoon the brioche and leek bake onto serving plates and place the pork belly alongside. Pour over the gravy.