First of all, make sure the strips of belly pork are absolutely dry by patting them with kitchen paper.
Then rub each one all over with olive oil and season with freshly milled pepper (but no salt because of the sauce). Now pop them into the roasting tin, tucking the chopped onion in among them and sprinkling them with a few more drops of oil. Place the tin on the highest shelf in the oven and let them cook for 30 minutes. Meanwhile, make up the barbecue sauce simply by whisking all the sauce ingredients together until blended thoroughly.
When the pork has been cooking for 30 minutes, pour off any excess oil from the roasting tin, then pour the barbecue sauce over the pork and cook for a further 25 minutes, basting frequently.
Pre-heat the oven to gas mark 6, 400°F (200°C).