Heat oil in a large heavy pot over medium-high heat. Add London broil and cook until well browned on all sides. Add mushrooms, Guinness, broth, steak sauce, onion and mustard. Cook, covered, over low heat for 1 hour. Crack lid and cook for 1 hour more. Stir in celery. Preheat oven to 400F Spoon beef mixture into 6 oven-safe bowls. Top each with a square of puff pastry, stretching to cover top. Bake for 20-25 minutes or until pastry is golden brown.