Place the cubed beef and diced onion into the slow cooker and spread them into an even layer.
Add the cream of mushroom soup, beef broth, Worcestershire sauce, garlic powder, salt, black pepper, and dried parsley if using. Stir gently until everything is combined.
Cover and cook on Low for 6 to 8 hours or on High for 4 to 5 hours, until the beef is very tender.
During the last 15 to 30 minutes of cooking, stir in the sour cream. Cover and let it warm through until the sauce is smooth and creamy.
Cook the egg noodles according to package directions. Drain and serve the beef stroganoff spooned over the noodles.