Place the beef stew meat, sliced onion, and sliced mushrooms in the slow cooker. Add the beef broth, Worcestershire sauce, garlic powder, salt, and black pepper. Stir to combine.
Cover and cook on low for 7 to 8 hours or on high for 4 to 5 hours until the beef is very tender.
In a small bowl, whisk the flour into the sour cream until smooth. Stir this mixture into the slow cooker and cook on high for 15 to 20 minutes until the sauce thickens.
While the sauce thickens, cook the egg noodles in a large pot of boiling salted water according to package directions. Drain and return them to the pot.
Add the butter to the hot noodles and toss until the butter melts and coats the noodles evenly.
Spoon the buttered noodles into bowls and top with the beef stroganoff. Sprinkle with chopped parsley before serving.