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PdfIn a small bowl combine the mustard, 1 ½ teaspoons of the sugar, a pinch of 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- to 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, sauté for 20 to 30 minutes, or until the vegetables are soft. Drain them in a sieve.
With a large, sharp knife cut the fillet across the grain into ¼-inch-wide strips. Heat 2 tablespoons of oil in another heavy 10- to 12-inch skillet over high heat until very hot but not smoking. Drop in half the meat and, stirring 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 skillet containing 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 ½ teaspoon of sugar and reduce the heat to low. Cover the pan and simmer 2 or 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.