Add the beef and onion: Place the cubed beef and diced onion into the slow cooker.
1½ pounds stewing beef or chuck roast, 1 small onion
Layer in the mushrooms: Scatter the sliced mushrooms on top of the beef.
Mix the sauce: Stir in the cream of mushroom soup, beef broth, Worcestershire sauce, garlic powder, salt, pepper, and parsley until everything is combined.
1 can, 1 cup beef broth, 1 tablespoon Worcestershire sauce, 1 teaspoon garlic powder, ½ teaspoon salt, ½ teaspoon black pepper, 1 teaspoon dried parsley
Slow cook it: Cover with the lid and cook on Low for 6–8 hours or High for 4–5 hours until the beef is fork tender.
Finish with sour cream: Stir in the sour cream during the last 15–30 minutes of cooking so it melts smoothly into the sauce.
¾ cup sour cream
Cook the noodles: While the stroganoff finishes, boil the egg noodles according to package directions. Drain well.
8 ounce egg noodles
Serve: Spoon the creamy beef stroganoff over a pile of noodles and enjoy.