Carrots, celery and onionsâ€”diced small for a wonderful blend of flavorsâ€”are a rustic complement to the garlicky lamb.
Preheat oven to 205Â°C (6 Gas). Trim any excess fat and membrane from the leg of lamb and discard. Brush oil on the lamb and sprinkle with salt and pepper. Place lamb on a rack in a shallow roasting pan. Insert a meat thermometer into thickest portion of lamb, making sure the thermometer doesn't touch the bone. Roast, uncovered, for 30 minutes.
Reduce oven temperature to 165Â°C (3 Gas). Combine undrained tomatoes, onions, carrots, celery, rosemary and garlic in a large bowl. Spoon into the roasting pan around the meat.
Roast 45 to 55 minutes more or until meat thermometer registers 55Â°C (n/a Gas) for medium-rare (or 60 to 70 minutes more or until meat thermometer registers 65Â°C (n/a Gas) for medium), stirring vegetables once. Cover meat with foil and let stand 15 minutes. The temperature of the meat after standing should be 65Â°C (n/a Gas) for medium-rare or 70Â°C (n/a Gas) for medium. Stir vegetables and serve with lamb. If desired, garnish with rosemary sprigs.Â