Prepare the slow cooker with nonstick cooking spray.
Place the stew beef into the bottom of the slow cooker.
In a medium-size mixing bowl, stir together diced onion, mushroom soup, beef broth, Worcestershire sauce, dry mustard, garlic powder, salt, and black pepper.
Pour the mixture over the beef and mix to combine. Top the beef mixture with the sliced mushrooms.
Dot the top of the beef mixture with cream cheese.
Cover and cook on low for 7 to 8 hours or on high for 5 to 6 hours.
Stir in sour cream until blended. Fold in the egg noodles. Taste for seasoning, adjust seasoning if needed.
Sprinkle with chopped fresh parsley, optional garnish. Ready to serve.