Place spice rub ingredients in a mortar and pestle and blend.
Rub spice mix into the skin of the pork belly and drizzle skin with olive oil.
Pour BBQ sauce into a roasting tray, and then place the pork belly onto the roasting tray and baste the bottom part of the pork belly with the sauce, avoiding the skin. Leave to sit uncovered in fridge for 2 hours or overnight to marinate and allow skin to dry out.
Place onion wedges on base of roasting pan. Sit the pork on top. Add ½ cup of water to the pan and cover the pork with foil. Roast pork belly for 2 ½ hours or until tender. Remove foil and cook in hot oven at 220 °C for 30 minutes. This will help crisp up the skin.
Cut pork belly into slices and drizzle with pan juices and serve with cooked vegetables.