Slice the beef into thin strips approx. 1 1/2" 2" in length. Heat a large cast iron skillet over high heat and add the meat a few stripos at a time to sear the meat. Remove the meat from the heat and set aside. Reduce the heat in the skillet to medium and melt the butter. Add the onion, saute', until soft . Raise the ehat to med-high, add the mushrooms, saute;, stir frequently, cook for 15 20 minutes. Lower heat to med-low sprinkle in the flour, stir well for 1-3 minutes. Stir in the stock, cream, sour cream, & mustard. Cover, reduce heat to low and simmer for for 5-7 minutes. DO NOT ALLOW TO BOIL! Return the meat to the skillet, mix with sauce, stir in the dill & parsley, and serve.