Chop the steak into thick slices. Marinate with salt and pepper. Cut mushrooms into quarters, and chop the onion into small dice.
Add 2 tbsp of olive oil and 2 tbsp of butter. Once the butter is melted, add steak into the pan. Pan fry it until one side of beef is caramelized, flip and fry the other side.
Once both surfaces are caramelized, take the beef out of the pan. Add 1 tbsp of butter into the pan, then saute onion and mushroom. Season the mixture with salt, garlic, and black pepper.
Add 2 tbsp of flour into the pan, and stir well. Add 3 cups of beef broth into the pan. When it is boiling, add 2 tbsp of white wine into the pan. Simmer for 10 minutes.
Once the sauce is reduced, add beef into the pan and stir well. Add 1 tbsp of dijon mustard. Remove the pan from the stove.
Add 1/3 cup of sour cream into the pan, stir well with the sauce. Garnish with some chopped chives, and serve with egg noodles. Enjoy!