Pre-heat the oven to 190°C. Make a few incisions in the lamb with a sharp knife and stuff with the sliced garlic and sprigs of rosemary. Rub the olive oil all over the leg and season well.
Transfer to a roasting tray along with the unpeeled garlic cloves and cover the leg loosely with aluminium foil. Roast for 35 minutes, then remove the foil and roast for another 50-60 minutes depending on the desired degrees of cooking.
Remove from the oven when ready and cover loosely again with foil and allow to rest for 10-15 minutes. Transfer to a serving plate and spoon any accumulated juices over the leg. Sprinkle the crushed red peppercorns on top and garnish with fresh rosemary sprigs.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.