Pre-heat oven to 400 F.
Score the pork belly skin, taking care not to cut down to the meat, then rub generously with salt and pepper.
The secret to crispy crackling is to make sure the skin is dry, the salt aids in that.
Place the pork belly in a roasting pan and add the quartered onions.
Put the pork belly in the oven and allow to roast for 30 minutes at 400 F, then turn the heat down to 320 F and roast for 25 minutes per 1/2 lb.
When the pork belly is cooked through, switch the grill/broiler on and allow the skin to crisp.
Keep an eye on it as it can burn easily.
Remove from the oven and allow to rest for 20-30 minutes then slice and serve.