Preheat oven to 400 degrees F. 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 325 degrees F. Combine undrained tomatoes, onions, carrots, celery, rosemary, and garlic in a large bowl. Spoon into the roasting pan around the meat.

3. Roast 45 to 55 minutes more or until meat thermometer registers 135 degrees F for medium-rare doneness or 60 to 70 minutes more or until meat thermometer registers 150 degrees F for medium doneness, stirring vegetables once. Cover meat with foil and let stand 15 minutes. The temperature of the meat after standing should be 145 degrees F for medium-rare or 160 degrees F for medium. Stir vegetables and serve with meat. If desired, garnish with rosemary sprigs. Makes 8 to 10 servings (3 ounces cooked meat and 1/2 cup vegetables per serving)