In a small bowl combine the mustard, 1 1/2 tsp of the sugar, a pinch of the salt and enough hot water to form a thick paste. Let the mustard rest at room temperature for 15 minutes. Heat 2 tablespoons of the oil in a heavy 10"-12" skillet over high heat untl a light haze forms above it. Drop in Onions and mushrooms, cover the pan and reduce heat to low. Simmer and stir for 20-30 minutres or until vegetables are soft. Drain them in a seive, discard the liquid and return mixture to 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 it with the grain into 1/4" wide strips. Heat 2 tablespoons of oil in another heavy skillet over high heat until hot but not smoking. Drop in half the meat and tossing the strips constantly, fry for 2 minutes or until meat is lightly browned.. Transfer meat to vegetables and do the remaining meat. When all meat is done, and added to the vegetables, stir in remaining salt, pepper and mustard paste. Stir in the sour cream a tablespoon at a time, then add the remaining 1/2 tsp sugar and reduce heat to low. Cover the pan and simmer for 2-3 minutes. Correct seasoning. Trans fer to heated serving platter and optionally add straw potatos on the top as a garnish