Halve the garlic cloves lengthwise. Use the tip of a sharp knife to make several small incisions in the lamb. Push the pieces of garlic and small sprigs of rosemary into the incisions. Season with salt and pepper.
Heat the butter with 1 tablespoon of olive oil in a large, roasting tin or oven-proof dish. Brown the lamb on all sides in about 5 minutes. Insert the meat thermometer into the centre of the thickest part of the leg. Place the leg of lamb in the oven. After 35 minutes, increase the temperature to 170 °C. Roast the lamb for approx. 40 minutes or until the thermometer reads 60 °C (rosé) or 65 °C (well done).
Meanwhile, scrub the sweet potatoes and cut them into wedges. Place on a baking tray lined with baking paper. Sprinkle with 3 tablespoons of olive oil. Place in the oven and bake for approx. 25 minutes until golden brown and cooked.
Slice the sweet peppers and courgette into strips. Add to the baking tray with the potato after 10 minutes. Sprinkle with the rest of the olive oil and season with salt and pepper.
Leave the lamb to rest covered for 10 minutes after you remove it from the oven. Slice the meat thinly and serve with the sweet potato and roast vegetables.
Tip: If you can’t heat your oven dish or roasting tin on the hob, brown the lamb in a frying pan and then transfer it to suitable oven dish or roasting tin.
