Preheat the oven to Gas Mark 8 or 240°C or 440°F.
Let the butter soften at room temperature. Peel the loose skin from the garlic heads/bulbs and soak them in warm water for 2 minutes before draining. Grease a large roasting pan with butter and drizzle the olive oil around it. Put the leg of lamb into the pan. Season the lamb with salt and pepper and place small sprigs of thyme and rosemary on the meat. You can prick small holes in the meat and insert the rosemary if you prefer.
Pour 150ml of water into the base of the pan and add the garlic heads/bulbs. Cook for approximately 1 hour regularly basting the lamb in its own juices. Check to see if the meat is cooked. If it needs a little more return to the oven for a further 10 minutes. Reduce cooking time if leg of lamb is smaller than 1.7kg.
Leave to stand for 10 minutes in a warm place, carve the lamb, garnish with the garlic bulbs which will be soft, sprinkle some fresh thyme and rosemary and serve with roast potatoes, mint sauce and seasonal vegetables.
*A 1.7kg lamb will take approximately 1hr - 1hr 10 minutes - less if you like your lamb rare or if you are cooking a smaller joint.
Also, this dish is quite indulgent so shouldn't be eaten too often, but it's perfect for those special occassions!
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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.