Sprinkle flour over the mushroom mixture. Gradually stir in beef broth, stirring constantly to thicken.
Lower the heat and stir in sour cream and mustard. Return beef to the pan and heat through. Garnish with parsley before serving.
Notes
Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat gently on the stove, adding beef broth if needed to maintain creaminess.