Place the onions on the base of a roasting tray (their purpose is, as well as flavor, to stop the belly sticking). Lay the belly on top. Rub the skin with a little oil and then the salt. Place in a medium to hot oven for approximately 1½–2 hours; keep an eye on it so it does not burn. If you’re anxious that the skin is not crisping up, you can start or finish the belly under the grill.