Place beef, onion, garlic, and mushrooms in the slow cooker.
Add beef broth, Worcestershire sauce, Dijon mustard, salt, and pepper.
Cover and cook on LOW for 7-8 hours or HIGH for 4-5 hours until beef is tender.
Whisk sour cream with flour or cornstarch until smooth.
Stir sour cream mixture into slow cooker 30 minutes before serving. Cook on HIGH until thickened.
Serve over egg noodles, rice, or mashed potatoes. Garnish with parsley if desired.
Notes
For deeper flavor, add a splash of white wine or sherry with the broth. Stir in Greek yogurt off heat if substituting for sour cream to prevent curdling.