2 lbs. beef sirloin or tenderloin
5 tbsp. butter
1 med. onion, peeled and chopped
1/2 lb. fresh mushrooms, cleaned and sliced
3 tbsp. tomato paste
1 tbsp. prepared mustard
1 c. beef bouillon
Pinch of salt
Salt and pepper to taste
2 tsp. flour
1 c. sour cream, room temperature

Remove any fat from meat and cut crosswise into strips about 1/2 inch thick and 3 inches long. Heat 2 tablespoons of butter in a skillet. Add some of the meat and brown quickly. Remove to plate. Add more butter and brown rest of the meat, also removing to plate. Saute onion in drippings. Add mushrooms and more butter, if needed and saute 4 minutes. Stir in tomato paste and mustard. Add bouillon, sugar, salt, and pepper. Bring to a boil, lower heat, and cook slowly (covered) for 10 minutes. Add meat and cook 5 minutes longer. Combine flour and sour cream and add to sauce. Leave on stove 3 to 5 minutes over low heat. Serve over buttered egg noodles or rice. Serves 4.