Preheat your oven to 180°C (350°F). Pat the pork belly dry with paper towels, then use a sharp knife to score the skin deeply in a crosshatch pattern, being careful not to cut into the meat. Rub a generous amount of salt into the skin and meat, focusing on the skin to help draw out moisture. Let the pork sit uncovered in the fridge for at least 1 hour, or overnight if possible, to dry out the skin further.
While the pork is chilling, peel and core the apples, then chop them roughly. Place the apples into a saucepan, add a splash of water, and cook over medium heat until they soften and start to break down—about 15 minutes. Mash the cooked apples with a fork or blend until smooth. Stir in lemon juice or vinegar to brighten the flavor. Keep warm.
Position the pork belly on a roasting rack set inside a baking tray, skin side up. Roast in the oven for about 1.5 hours, until the meat is tender and the skin is golden and crackling. During roasting, baste the meat with rendered fat if needed, and in the last 10-15 minutes, increase the oven temperature to 220°C (430°F) to help the skin crisp up. Keep a close eye to prevent burning.
Once cooked, remove the pork from the oven and transfer to a cutting board. Let it rest for 10-15 minutes, tented loosely with foil, to allow juices to settle. Meanwhile, reheat the apple puree gently if cooled, and taste it, adding more lemon or salt if desired.
Slice the pork belly into thick, even pieces, ensuring you get a good balance of crispy skin and tender meat. Plate the slices and spoon a generous amount of bright apple puree alongside. Drizzle with a little of the rendered pork fat or juices for extra flavor, and serve immediately to enjoy the contrast of textures and flavors.
