Place the belly skin side up on an oven rack and pat the skin dry with kitchen paper. Place uncovered in the refrigerator to dry out overnight (or at least for 3-6 hours). Remove from the fridge and preheat the oven to 230 C. Brush the skin side with extra virgin olive oil. In a pestle & mortar or a small blender, crush the salt with about 5 of the sage leaves (chop the leaves before grinding) until you have a pale green salt mix. Use about 2 teaspoons of this mixture to rub into the skin of the belly, making sure to get into the score lines. Place the rack over a regular baking tray lined with baking paper, then roast at 230 for 30 minutes to get the skin puffed up and crispy. While the skin is crisping, prepare a deep rectangular roasting tray by brushing the bottom with olive oil. Add the onion wedges and apples, a handful of sage leaves and season with salt & pepper. Drizzle all over with olive oil. When the 30 minutes roasting time of the belly is up, remove from the oven and turn down the heat to 170 C. Transfer the rack with the belly from the baking tray to the roasting dish, so that the pork belly’s drippings will land over the apples and onions. Return to the oven for 2 hours at 170 C. The skin side should now be golden and puffed and the meat cooked but still juicy. Remove from the oven, let it rest for 10-15 minutes, then slice and serve hot with the apples and onions, the extra sage salt, and a crisp green salad.