Whisk together beef broth, Worcestershire sauce, mustard, and tomato paste, then pour over beef and onions in the slow cooker.
Cover the slow cooker and cook on low for 7-8 hours, adding mushrooms during the last 30 minutes.
Stir sour cream blended with flour into the slow cooker until incorporated. Serve hot over noodles or rice, garnished with parsley.
Notes
This dish stores well and can be refrigerated for up to three days or frozen for a longer period. Always heat gently before serving to maintain the creamy texture.