To roast the pork, use a sharp knife or box cutter to score the skin and fat of the pork against the grain of the meat. Do not cut through the meat. Keep in mind the more scores you make the crispier the crackling. Place the pork on a wire rack set on a large, heavy, rimmed baking tray. Refrigerate uncovered for 12 hours. Let stand at room temperature for 1 hour before roasting.
Position a rack in the centre of the oven and preheat the oven to 250°C. Rub the salt flakes into the scored skin and fat. Season the rest of the pork meat with more salt. Roast the pork for 35 mins or until the skin of the pork is brown and beginning to crisp. Reduce the oven to 150°C.
Rub 2 tsp of oil over the outside of the pumpkin and place it into the tray with the pork. Return the tray to the oven and roast the pumpkin and pork for 40 mins or until the pumpkin is just tender enough to cut through easily. Transfer the pumpkin to a cutting board. Continue to roast the pork for 20 mins (1 hour in total at 150°C) or until the pork is tender. Set the pork aside to rest for 20 mins.
Increase the oven temperature to 250°C and place a heavy baking tray in the oven to preheat. Scoop out and discard the seeds and innards from the pumpkin and cut the pumpkin into 8 wedges. Brush both sides of the pumpkin wedges with 2 tbs oil and sprinkle with salt. Place the wedges evenly apart on the hot baking tray and roast for 10 mins on each cut side or until caramelised.
Meanwhile, to make the adobo sauce, heat a medium, heavy saucepan over high heat. Add the remaining 1 tbs oil, then add the onion, chillies, orange peel and garlic. Cook, stirring occasionally, for 6 mins or until the vegetables are charred in spots. Stir in the paprika, salt, peppercorns, cumin, thyme and allspice. Stir for 1 min to toast the spices. Add the tomato paste and stir for 2 mins or until the mixture has turned a dark red colour. Add the stock, vinegar and 2 tbs of the orange juice. Simmer for 5 mins or until the liquid has reduced by one-quarter. Strain the sauce through a fine-mesh sieve, using the back of a ladle to extract as many solids as possible. Season the sauce with salt and more orange juice, if necessary. Cover and keep warm until ready to serve.
Slice the pork belly and divide among serving plates. Serve with the roasted pumpkin, adobo sauce and coriander.