Preheat the oven to gas 4, 180°C, fan 160°C. Cut the onion, carrot and celery into rough chunks, put into a large roasting tin, then add 150ml (¼ pint) of wine.
Finely chop the garlic and parsley together. Then mash them into the butter, add seasoning and lemon zest.
Season the lamb then, using a sharp knife, make long and deep cuts around the leg of the lamb, going about 2.5cm (1in) into the flesh. Push the parsley mix into the cuts with your hands. Sit the lamb on the vegetables, then roast for 1 hour 40 minutes (or follow the rule: 40 minutes per kg – 20 minutes per lb – plus an extra 20 minutes). The lamb should be golden on the outside and just pink near the bone.
Take the lamb out of the roasting tin and set aside. Remove any excess fat from the tin with a spoon, then add the remaining wine to the juices. Put the tin on the hob and boil the juices until reduced by half. Add the stock and sugar, and simmer for 10 minutes. If you like thick gravy, mix the cornflour with a splash of water, stir into the gravy, then whisk until smooth. Season, then strain the gravy into a jug. Carve the lamb and serve with vegetables and gravy.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.