Heat the oven to 240C/220C fan/gas 9. Remove any packaging from the pork, retaining the string, and pat dry with kitchen paper – you want the skin to be as dry as possible. Score deep cuts into the skin in a diamond pattern using a sharp knife (you want to cut into the fat). If the skin is already scored, make sure each cut is dry and can be easily opened. Rub the oil into the skin, rubbing it well into each cut, then sprinkle the salt over in an even layer, rubbing this into the cuts, too.

Scatter the onion slices over the base of a large roasting tin and sit the pork joint on top. Roast for 20-30 mins, or until the skin starts to blister and bubble. Put the kettle on to boil.

Pour half a kettle of boiling water around the joint to cover the base of the tin, ensuring it doesn't drip onto the skin. Reduce the oven temperature to 180C/160C fan/gas 4. Roast for another 2 hrs 30 mins, or until the meat under the skin is very tender when pressed. There’s a lot of fat in pork shoulder, so cooking at a low temperature this way ensures the meat doesn’t dry out – if needed, it can cook for up to 3 hrs.

Turn the oven back up to 220C/200C fan/gas 7 and continue to roast the pork for 20-25 mins until the skin is very crisp and has started to harden – it should feel brittle when tapped with a knife. If needed, cook for 10 mins more to achieve this. Remove the pork from the oven and rest for 20 mins in the tin, loosely covering with foil. Remove the crackling in large pieces, then break into smaller chunks. Carve the meat and serve alongside the crackling and your favourite sides.