Preheat oil 5 minutes at 375 deg. Meanwhile, wipe 1/2-inch thich steak with paper towels to remove excess moisture. Mix dry mustard, salt and pepper. Sprinkle over one side of steak; Rub in. Brown steak in hot oil, about 1 1/2 minutes on each side. Sprinkle with chives, lemon juice and worcester- shire. At tableside, pour heated cognac over steak. Ignite. When the flame goes out, transfer steak to serving platter. Spoon pan juices over meat and serve.