Quarter the onions. Rub the inside of the belly with the marinade, add onions and seal the whole thing. Now scratch the skin (be careful not to damage the meat) and then massage in with the marinade.
Wrap the whole thing in foil and let it soak in for at least a day.
Now heat up the grill properly (preferably with beech wood) and start the grilling process.
Brush with oil every now and then and grill for about 9 hours.
Be careful with the beer consumption during the grill process, 9 hours can be long.