Fry the onion in butter until softened, then add chopped liver and cook briefly.
Remove from heat, cool, and mix in remaining stuffing ingredients.
Spread stuffing over the lamb and tie it into a roll with kitchen string.
Preheat the oven to 180°C.
Place lamb on a roasting tray, cover with foil, and roast for 135 minutes.
Remove foil during the final 30 minutes of roasting.
Rest the lamb in a warm place.
Fry the onion in dripping, then stir in flour until absorbed.
Pour in port and simmer until the liquid reduces by half.
Add stock and reduce by half again.
Whisk in jelly, seasonings, nutmeg, and browning to finish the gravy.