Cut the meat into slices and then into strips. If the meat is cut too thin it hardens in cooking. Sprinkle with salt and pepper, and allow to stand.
Brown the flour and half the butter over heat and work to a paste gradually by adding the beef stock, tomato puree and mustard. Bring to a boil and allow to stand.
Twenty minutes before serving, fry the onion rapidly in the remaining butter with the meat, turning with a spatula, for 5 minutes and cooking only enough at a time to cover the bottom of the pan. Add the thinly sliced mushrooms.
When the meat is ready, add sour cream to the sauce. Place together with the meat, mushrooms and onion in a firmly lidded pan. Bring to near boiling point before serving.