1 tbsp. powdered mustard
1 tbsp. sugar
2 tsp. salt
4-5 tbsp. vegetable oil
4 c. thinly sliced onions, separated into rings (3 lg. onions)
1 lb. fresh mushrooms, thinly sliced lengthwise
2 lb. fillet of beef, trimmed of all fat
1 tsp. freshly ground black pepper
1 pt. sour cream

In a small bowl combine the mustard, 1 1/2 teaspoon of the sugar, a pinch of the salt and enough hot water (perhaps a tablespoon) to form a thick paste. Let the mustard rest at room temperature for about 15 minutes.

Heat 2 tablespoons of the oil in a heavy 10-12 inch skillet over high heat until a light haze forms above it. Drop in the onions and mushrooms, cover the pan and reduce the heat to low. Stirring from time to time, simmer 20-30 minutes, or until the vegetables are soft. Drain them in a sieve, discard the liquid and return the mixture to the skillet.

With a large, sharp knife cut the fillet across the grain into 1/4 inch wide rounds. Lay each round on a board and slice it with the grain into 1/4 inch wide strips. Heat 2 tablespoons of oil in another heavy 10-12 inch skillet over high heat until very hot but not smoking. Drop in half the meat and, tossing the strips constantly with a large spoon, fry for 2 minutes or so until the meat is lightly browned. With a slotted spoon transfer the meat to the vegetables in the other skillet and fry the remaining meat similarly, adding additional oil if necessary. When all the meat has been combined with the vegetables, stir in the remaining salt, pepper, and the mustard paste. Stir in the sour cream, a tablespoon at a time, then add the remaining 1/2 teaspoon of sugar and reduce the heat to low. Cover the pan and simmer 2-3 minutes, or until the sauce is heated through. Taste for seasoning.

To serve, transfer the contents of the pan to a heated serving platter, and if you like, scatter straw potatoes over the top.