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["In a small bowl combine the mustard, 1 1/2 tsp of the sugar, a pinch of the salt and enough hot water (about a tablespoon) to form a thick paste.", "Let the mustard rest at room temperature for about 15 minutes.", "Heat 2 tbsp of the oil in a heavy 10"" to 12"" skillet over high heat until a light haze forms above it.", "Drop in the mushrooms and onions, cover the pan, and reduce to low heat.", "Stirring from time to time, simmer 20-30 minutes, or until 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"" wide rounds.", "Lay each round on a board and slice them into 1/4"" wide strips.", "Heat 2 tbsp of oil in another heavy 10"" to 12"" skillet over high heat until very hot, but not smoking.", "Drop in half of the meat and, tossing the meat 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 the same way, adding more oil if needed.", "When all the meat has been combined with the vegetables, stir in the remaining salt, pepper and mustard paste.", "Stir in the sour cream, a tablespoon at a time, then add the remaining 1/2 tsp of sugar and reduce to low heat.", "Cover the pan and simmer 2-3 minutes, or until the sauce is heat through.", "Taste for seasoning.", "To serve, transfer to a heated serving platter, and if you like, scatter straw potatoes over the top."]
