Spread Leggo’s Pesto evenly over lamb and sprinkle with rosemary. Place lamb on a roasting rack in a roasting pan and pour water into the pan.
Loosely cover roast with foil and place in a preheated oven at 200°C for 1 hour. Remove foil and continue cooking for another 40 minutes. Cover lamb with foil and set aside to rest for 10 minutes before slicing.
Serve with roasted vegetables.