1. Cut the meat into stripes, season with salt and pepper.
2. In a large skillet heat oil, add beef strips and cook until brown, about 2-3 minutes on each side. If your skillet to small, cook in two batches. Remove from the pan and set aside.
3. Reduce the heat to medium. Add to the same pan butter, chopped onion and sliced mushrooms. Sauté for 5-7 minutes, until soft. Add garlic and sauté for 1 minute more.
4. Add flour and cook for 1 minutes, stirring constantly. Pour in the broth, stir and scrape the bottom of the pan. Add the cooked beef and bring to a simmer. Reduce to medium-low and simmer for 15 minutes.
5. Temper the sour cream: pour few tablespoons of the hot sauce to the sour cream and stir until combined. Pour back to the pan. Add mustard, season with salt and pepper, stir. Simmer for 10-15 minutes, until slightly thickens. Turn the heat off, add chopped chives and stir.
6. Serve with noodles/pasta or rice.