Method. 1  Preheat the oven to 220ºC. 2  Brush the roast lightly with oil and season with salt and pepper. 3  Place the lamb on a rack in a roasting dish. 4  Roast for 15 minutes for rare, 25 minutes for medium, or 30 minutes for well done.5  For ease and accuracy use a meat thermometer. Place the lamb on a rack in a roasting dish. 2  Roast for 15 minutes for rare, 25 minutes for medium, or 30 minutes for well done. 3  For ease and accuracy use a meat thermometer.