Preheat the oven to 400ºF.
Line the bottom of a roasting pan with heavy-duty aluminum foil and fit it with a rack.
With the butt end of the roast facing you, make about a dozen slits in the fleshy part of the meat with a sharp knife. In a bowl, mix together the garlic, 3 tablespoons of the rosemary, 1 teaspoon of the salt, 1/4 teaspoon of the cayenne, 1/4 teaspoon of the black pepper, and 3 tablespoons of the mustard to make a paste. Using your fingers, insert equal amounts of the paste into the holes. Tie the leg crosswise with the kitchen twine, at 3-inch intervals. Season the outside of the lamb with 1 teaspoon of the salt, 1/2 teaspoon of the black pepper, and the remaining 1/2 teaspoon cayenne.
Heat 3 tablespoons of the vegetable oil in a pan large enough to accommodate the lamb over high heat. Sear the lamb for about 3 minutes on all sides. Remove the lamb from the skillet and set aside. Combine the bread crumbs, the remaining 3 tablespoons rosemary, the rub, and 1/4 cup of the oil in a bowl. Rub the top and sides of the lamb with the remaining 1/4 cup mustard. Using your fingers, press the bread crumb mixture evenly and firmly onto the mustard-covered area. Place the lamb on the rack in the roasting pan and roast for 1 hour.
Toss the potatoes, carrots, onions, and celery with the remaining 2 teaspoons salt, remaining 1 teaspoon black pepper, and 1 tablespoon oil. Remove the pan from the oven and place the vegetables on the rack around the lamb. Return the pan to the oven and roast for about 1 hour 45 minutes, or until the internal temperature of the lamb registers 160ºF. on a meat thermometer for well done. If you prefer the lamb to be rare, roast for about 1 hour, or until a meat thermometer registers 140º to 145ºF. Remove the roast and let it rest for 20 to 30 minutes before carving.
Serve with the vegetables.
Yield: 10 to 12 servings