For the stuffing, fry the onion in a frying pan in the butter until softened. Add the lambâ€™s liver and cook for a few minutes. Remove from the heat and cool before adding the remaining stuffing ingredients.
Spread the stuffing across the length of the boned lamb, tie it into a roll with kitchen string. This can be done the day before and refrigerated overnight.
Preheat the oven to 180C/350F/Gas 4.
Put the lamb on a roasting tray, cover with foil and roast for 2Â¼ hours. Remove the foil for the final half hour of cooking.
Place the lamb in a warm place to rest.
For the gravy, heat the dripping in a pan and fry the onion until softened. Add the flour and stir until all the flour is absorbed. Pour in the port and cook until the volume of liquid has reduced by half.
Pour in the stock and cook again until the volume of liquid has reduced by half again. Add the redcurrant jelly, salt, freshly ground pepper, nutmeg and gravy browning to taste.