Add beef and onion: Place beef cubes and diced onion into the slow cooker.
Add mushrooms: Add sliced mushrooms on top of the beef.
Add sauce ingredients: Add cream of mushroom soup, beef broth, Worcestershire sauce, garlic powder, salt, pepper, and parsley. Stir gently to combine.
Slow cook: Cover and cook on Low for 6–8 hours or High for 4–5 hours until the beef is fork tender.
Add sour cream: Stir in sour cream during the last 15–30 minutes of cooking. Allow it to warm through.
Cook noodles: Cook egg noodles according to package instructions and drain.
Serve: Spoon the beef stew over noodles and serve hot.
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