Brine your pork belly piece for 3 days, rinse, then score the skin gently with a sharp knife (a boxcutter is excellent for this purpose).
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.
When cooked you should have crispy skin on top of soft and giving fatty flesh. Lift off the onions and serve.