Add the beef and diced onion to the bottom of your slow cooker.
Layer the sliced mushrooms over the beef.
Add the cream of mushroom soup, beef broth, Worcestershire sauce, garlic powder, salt, pepper, and parsley. Stir gently to combine.
Cover and cook on Low for 6–8 hours or High for 4–5 hours, until the beef is fork-tender.
During the last 15–30 minutes of cook time, stir in the sour cream and let it warm through completely.
Meanwhile, cook the egg noodles according to package directions, then drain.
Serve the stroganoff hot, spooned generously over the noodles.