1Cut steak into strips 2 1/4 inch long. In large skillet, saute mushrooms and onions in 3 tablespoons butter until golden brown. Remove from skillet. Add remaining 1 tablespoon butter, brown meat on all sides for 15 minutes. Dissolve bouillon cubes in boiling water, pour over meat. Add tomato paste, mustard and salt. Simmer 45 minutes until tender. Combine flour and water. Slowly pour in
meat mixture. Cook, stirring constantly, until mixture comes to a boil. Reduce heat. Add mushrooms, onions and sour cream. Heat but do not boil. Serve on hot rice or egg noodles.