Cut beef diagonally across grain into thin slices. In 3 1/2- or 4-quart slow cooker, place beef. Add onion.
In medium bowl, mix soup, cornstarch, brown sugar, Italian seasoning, red pepper and Worcestershire sauce until smooth. Pour over beef and onion.
Cover; cook on Low heat setting 10 to 12 hours.
To serve, place 1 slice of bread on each individual plate. Top each with slice of cheese. Spoon about 3/4 cup beef mixture over each. Sprinkle with parsley.