Heat the oven to 150C/fan 130C/gas 2. Mix all the stuffing ingredients well and season.
Lay 4 pieces of kitchen string on a worksurface. Put the pork skin-side down on the string. Spread over the stuffing. Bring up the sides, reshape the pork into a joint and tie the string so the stuffing is secured. Season the skin with the sea salt.
Cut the onions into thick slices and put in the bottom of a roasting tin to form a trivet. Sit the pork on top and pour 1/2 a mug of water in the bottom of the tin. Cover the whole tin tightly with foil and cook for 3 hours.
Remove the foil and turn up the oven to 200C/fan 180C/gas 6. Cook for a further hour, or until the skin has crackled properly. Take the pork from the oven, remove from the tin and leave to rest, covered, for 30 minutes.
To make the gravy, put the roasting tin over a medium heat and skim away most of the fat. Add the flour and stir well, taking up all the bits in the pan and the fat. Gradually add the stock and wine, and increase the heat to a simmer, stirring all the time. Simmer for 10 minutes, then season if it needs it. Serve the pork with the gravy and trimmings.