Preheat the oven to 170C/325F/Gas 3.
For the pork belly, boil a kettle and pour the boiling water over the pork skin, discarding the water.
Pat the pork belly dry, using kitchen paper, then rub the salt and thyme leaves over the skin, pressing them into the score marks.
For the boulangère potatoes, mix the potato and onion slices together in a bowl and season, to taste, with salt and freshly ground black pepper. Transfer the mixture to a large buttered ovenproof dish and pour over the stock.
Transfer the dish to a rack in the lower part of the oven. Place the pork belly directly onto the shelf above the potatoes so that any fat from the pork belly drips onto the potatoes. Roast for three hours, or until the pork belly and potatoes are tender and golden-brown.
For crisp pork crackling, remove the potatoes from the oven and heat the grill onto its highest setting. Grill the pork belly for 10-15 minutes, or until the crackling is crisp. Remove from the grill and set aside.
Meanwhile, for the apple sauce, place the apples and water into a lidded saucepan, cover the pan with the lid and heat over a medium heat for 3-4 minutes, or until the apples have broken down to a purée. Stir in the butter and sugar, adding more sugar, if necessary, to taste.
To serve, carve the pork belly into thick slices and place onto four serving plates. Spoon the boulangère potatoes alongside and top with a dollop of apple sauce.