Preheat the oven to 425 degrees F. Place the lamb in a roasting pan. Brush with mustard. Sprinkle with herbes de Provence, salt and pepper. Roast 20 minutes, and then reduce heat to 325 degrees F. Cook lamb to desired degree of doneness, checking its progress with a meat thermometer, 75-90 minutes should yield medium rare in the middle. While this occurs, soak mushrooms in warm water 30 minutes, then drain well, slice, and then set aside. When cooked, place lamb on a platter and rest 10-15 minutes. Remove excess fat from roasting pan, add the wine and mushrooms and reduce by half. Add 2 cups of the beef stock and bring to a boil. In a bowl, mix the remaining stock with the flour until smooth. Whisk this mixture into the pan. Simmer until sauce thickens, about 5 minutes, then season to taste. Serve slices of lamb with a spoonful of the sauce.
Note: Dried morel mushrooms are available in the produce department.