This fabulous lamb roast is complemented by the addition of sweet caramelised onions. Serve with a zesty gremolata for a gorgeous vibrant centrepiece. Preheat the oven to gas 7, 220°C, fan 200°C. Drizzle a little oil over the lamb, and season. Put in a roasting tray and cook for 20 minutes.Heat the butter and the remaining oil in a wide pan. Fry the onion until soft and golden. Add the garlic and fry for a minute. Add the vinegar and let it bubble. Stir in the wine, thyme and rosemary, and simmer for 1-2 minutes more. Put the onions around the lamb, pour in the stock, and cover the tray with foil.Reduce the oven to gas 6, 200°C, fan 180°C and roast the lamb for 15 minutes per 500g (1Ib) – 1 hour and 15 minutes for a 2.5kg (5Ib) leg. The lamb will be just pink in the middle. For well-done meat, roast for 20 minutes per 500g. Baste the lamb with the cooking juices a few times during cooking, and then transfer to a serving plate to rest for 20 minutes.To make the gremolata, use a potato peeler to remove the peel from the lemon. In the middle of a chopping board, cut the peel into thin strips, top with the garlic, parsley and rosemary and finely chop everything together. Season, place in a dish and set aside.Serve the lamb with the onions and gremolata.See more Lamb recipes For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.