Take lamb out of fridge at least 1 hour before roasting.
Preheat oven to 240°C/475°F (220°C fan) with oven shelf in the middle.
Rub – Mix rosemary, garlic and olive oil.
Prepare lamb – Place lamb leg in a roasting pan. Coat with the rub using your hands, the sprinkle with the salt and pepper. Place garlic and rosemary sprigs underneath the lamb, garlic cut face up.
Roast lamb leg for 20 minutes. Turn the oven down to 200°C/400°F (180°C fan) and roast for a further 1 hour or until the internal temperature reaches 53°C/127°F for medium rare (Note 5 other doneness). Check first at 45 minutes – everyone's oven is different!
Rest – Remove lamb from oven. Transfer lamb to plate, loosely cover with foil and rest for 20 minutes to 1 hour (it will still be warm enough for serving!).