The night before you cook the pork, score the skin taking care not to cut through to the flesh. This is optional and makes no difference to the quality of the crackling, but it helps to season the pork fat beneath the skin. Leave the joint in the fridge uncovered overnight.
Around 15-20 minutes before you are ready to cook heat your oven as high as it will go (around 250°C or 480°F).
Cut the onion into slices around 1-1.5cm (½") thick and place them in a roasting tin.
Season the pork with salt and pepper, paying particular attention to rubbing it into the cuts in the pork skin, then place it on top of the onions.Pop the pork in the oven and cook it for 20-30 minutes or until the skin begins to blister.
Turn the heat down to 150°C or 300°F and allow the pork to cook for another 2-2½ hours.When the pork is done remove from the oven and allow to rest for 20-30 minutes.
Pour off all but a scant covering of fat from the roasting pan and place it over a high heat on the stovetop.
Pour in the cider scraping the pan as you go to deglaze it and reduce by half.
Add the chicken stock and reduce it by a further quarter to a third.
Transfer the stock gravy, including the onions to a blender and blitz until very smooth.
Pass the sauce through a fine mesh sieve into a 18cm or 7" saucepan and bring it to a boil.
Finally, whisk in the cold butter just before serving adding salt and pepper to taste.
Notes
If you want to roast potatoes add them to the roasting tin and baste them with the oil from the pork an hour before you remove the meat from the oven.Then turn the oven up to 180°C or 350°F whilst the pork is resting to crisp them up.