Cut steak into 6 serving-size portions. Coat with a mixture of flour, mustard, salt and pepper. Using a large frying pan, brown meat in half the butter and oil. Transfer to crockpot. Heat remaining butter and oil in frying pan. Saute onions, carrots and celery until glazed. Add tomatoes, Worcestershire and brown sugar. Heat, scraping up drippings. Pour over meat. Cover. Cook on low, 6 to 8 hours, or until tender. Serve meat with sauce spooned over. Sprinkle with parsley.