Place steak in slow-cooking pot. Combine sauterne, soy sauce, garlic, broth, catsup, mustard, and onion; pour over steak. cover and cook on low for 6 to 8 hours. Dissolve cornstarch in water; stir into pot. Add mushrooms. Turn control to igh; cover and cook on high 20 to 30 minutes or until mushrooms are done.