Melt butter and add mushrooms and onions and saute. Remove from pan and set aside. Melt more butter and cook meat until gray – do not brown. Dissolve 3 bouillion cubes in 1 cup boiling water and pour over meat. Add 2 tablespoons tomato paste and dry mustard, then cover and simmer for 45 minutes. Then mix together ½ cup water 2 tablespoons flour and add to mixture slowly, stirring constantly. (You may need more water.) Bring to a boil and then turn to simmer. Add mushrooms and onion mixture and heat. Then add sour cream and just heat and serve.