Bef Stroganov ( Original Russian style)

Step 1

In a small bowl combine the mustard, 1½ teaspoons of sugar, pinch of salt and enough hot water (perhaps a tablespoon) to form a thick paste. Let stand at room temperature for about 15 minutes. Heat 2 tablespoons of the oil in a heavy 10-12" 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 ¼" wide rounds. Lay each round on aboard and slice it with the grain into ¼: wide strips. Heat 2 tablespoons of oil in another heavy 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 oil as needed. When all the meat is combined with the vegetables, stir in remaining salt pepper and remaining mustard paste. Stir in the sour cream, a tablespoon at a time, then add the remaining ½" teaspoon of sugar, or until the sauce is heat to low. Cover the pan and simmer 2-3 minutes, or until sauce is heated through. Taste for seasoning