Cut steak into thin strips 2 + 1/2 inches long.
In large skillet,saute mushrooms and onion in 3 tbsp. butter until golden brown.
Remove from skillet. Add the remaining tbsps. of butter . Add beef and brown on all sides (about 15 minutes).
Dissolve bouillon cubes in boiling water; pour over meat.
Add tomato paste, mustard and salt.
Cover + simmer 45 minutes. until tender.
Combine flour and water + stir into meat mixture. Cook, stirring constantly until mixture comes to a boil.
Add the mushrooms, onions and sour cream .
Heat , but do not boil.
Serve over hot rice.
Prepare the rice according to package instructions.