Mix all the ingredients in a food processor and blend to a fine consistency.
Add 150ml olive oil and blend well, the blending method should be similar to that of making pesto.
Place the shoulders on the chopping board, skin down. Open the meat flat. Rub the porchetta mix into the meat. Fold it and tie it with butcher’s string or a cooking net.
Take a rectangular oven dish about 10cm deep, in which the pork shoulder would fit well. Fill with water to a height of about four centimeters. Seal with foil.
Cook for about an hour. The water in the dish will steam the skin and it will become soft.
After one hour, remove the foil. Reduce oven temperature to 180°C and cook for another hour. The core temperature should reach 80°C (with a probe). Add water if necessary.
For crackling, raise temperature to 245°C and cook until skin puffs up.