Preheat the oven to 220ºC, fan 200ºC, gas 7. To make the stuffing, heat 1 tablespoon of the oil in a frying pan over a medium heat and fry the onions for 8 minutes. Sprinkle over the sugar and fry for 4-5 minutes, until the onions are golden and caramelised. Stir in half the sage and season with freshly ground black pepper. Set aside to cool for 10 minutes.
Remove the string from the joint and score the skin with a sharp knife. Place, skin-side down, on a chopping board.
Cut at an angle through the flesh to open out the joint. Spoon the stuffing over the opened-out pork and sprinkle over the remaining sage. Roll up the joint and secure at intervals with kitchen string.
Rub the skin with the salt and the remaining oil. Put in a roasting tin and roast for 30 minutes. Reduce the oven temperature to 180ºC, fan 160ºC, gas 4 and roast for a further 1 hour 10 minutes. Increase the oven temperature to 220ºC, fan 200ºC, gas 7 for the last 10 minutes of cooking time for crisp crackling. Cover loosely with foil and rest for 15 minutes.
Pour the pork juices into a pan and stir in the flour to make a paste. Cook over a medium heat for 2-3 minutes before slowly pouring in 800ml water, whisking until you get a smooth, thick gravy. Simmer gently for 10 minutes.
Serve the pork and gravy with sides and trimmings