Layer the ingredients in the slow cooker in the following order: Onions/garlic, mushrooms and beef.
In a small bowl, combine the broth, mustard, Worcestershire sauce, salt, pepper and thyme. Pour over the beef in the slow cooker. Cook on LOW for 4 to 6 hours or on HIGH for 2 to 3 hours or until the beef is tender.
About 20 minutes before it's done, add the beef broth/cornstarch mixture to the slow cooker and stir to combine. Cook until thickened.
Stir in the cream cheese until dissolved. Stir in the sour cream and chopped fresh parsley and heat through. Add salt, pepper and mustard to taste.