Preheat the oven to 160°C (325°F; Gas Mark 3). With a sharp knife, trim all the fat and any cartilage from the lamb. Season the meat with salt and pepper. With a small knife, make slits about 1 inch deep all over the leg of Lamb.
Using most of the garlic and all the rosemary, push a garlic or a few rosemary, push a garlic leaves into each of the slits. In a large roasting pan, combine oil with sweet potatoes, parsnips, and remaining garlic to coat.
Move the vegetables to the side of the pan and place the lamb in the centre. Roast the lamb and vegetable for 1- 2 hours or until a thermometer inserted into the centre of the meat reads 84°C (160°F – for medium rare).
Turn the vegetables occasionally so that they cook evenly. Remove pan from oven. Transfer the vegetables to a serving dish and keep them warm.
Transfer the lamb to a carving platter, cover with foil and let stand for 5 minutes. Slice the roast lamb and serve with the vegetables.