Tenderize the beef lightly with a meat mallet. Salt and pepper the beef generously. Spread 3 Tablespoons of mustard on one side of the beef.
Cutting against the grain, cut into small, thin strips. You are wanting pieces slightly bigger than bite size.
Once the beef is cut, sprinkle with flour and put in a hot pan (heated over medium high heat with 2 Tablespoons oil or butter).
Sear the beef quickly, about three minutes. Don't walk away from it and watch your heat. It can burn easily so adjust your heat down if you think it is too hot.
Remove the beef from the pan and set aside.
Add 2 Tablespoons of oil to the same pan and add the onion. Cook for four minutes until soft. Reduce the heat to medium.
Add the mushroom and garlic and cook for another 3-4 minutes until the mushrooms are soft.
Add broth, tomato paste, butter, Worcestershire sauce, mustard and ground pepper. Mix until everything is well incorporated.
Add the beef back in and heat through.
Finally, turn the heat to low and mix in the sour cream. Mix well and let simmer for 3-5 minutes.
Add the parsley just before serving.