In 3- to 4-quart slow cooker, layer onion, carrots, mushrooms and beef. Sprinkle pepper over beef. In medium bowl, mix soup and tomato sauce; pour over beef.
Cover; cook on Low heat setting 8 to 10 hours.
Place beef on serving platter. With slotted spoon, remove vegetables from cooker and spoon over beef. Stir gravy in cooker with wire whisk to blend. Serve gravy with beef.