1 tbsp. dry mustard
1 tbsp. sugar
2 tsp. salt
4 to 5 tbsp. vegetable oil
4 c. thinly sliced onion, separated into rings
1 lb. fresh mushrooms, thinly sliced lengthwise
2 lbs. filet of beef, trimmed of fat
1 tsp. freshly ground black pepper
1 pt. sour cream
4 tbsp. cooking oil

In a small bowl, combine mustard, 1 1/2 teaspoon of sugar, pinch of salt, and about 1 tablespoon water to form a thick paste. Let mustard mixture rest at room temperature for 10 to 15 minutes. Heat 2 tablespoons oil in heavy skillet over high heat. Drop in onions and mushrooms, cover, and reduce heat to low. Stir and simmer 15 to 20 minutes; drain. Cut filet of beef across the grain into 1/4 inch rounds. Lay each round on cutting board and slice with the grain into strips 1/4 inch wide. Heat 2 tablespoons in another heavy skillet over high heat. Drop in meat and toss continuously, frying 2 to 3 minutes until brown. Transfer meat to vegetable skillet, stir well, then add remaining salt, pepper, and mustard paste. Stir in sour cream, 1 tablespoon at a time. Add remaining sugar (1/2 teaspoon); reduce heat. Cover and simmer 2 to 3 minutes. Serve with straw potatoes scattered over top or over rice.