Beef Strogonoff

In a small bowl combine mustard, 1 1/2 tsp of the sugar, pinch of salt and enough hot water to form a thick paste. Let mixture rest at room temperature for about 15 minutes. Heat 2 tablespoons of the oil in a heavy 10 - 12 inch frying pan over high heat, until a light haze forms over the oil. Add onions and mushrooms, cover and reduce heat to low. Simmer 20 - 30 minutes, stirring occasionally, until vegetables are soft. Drain them and return them to the pan. Slice beef across grain into 1/4 inch rounds, then slice into 1/4 inch strips. Heat 2 tablespoons of oil in another pan over high heat until very hot but not smoking. Add half the meat, tossing constantly until it is lightly browned (about 2 minutes). Add to vegetables and fry the other half of the meat, adding more oil if necessary. When cooked, add to the vegetables. Then add the remaining salt, pepper and the mustard paste. Increase heat to medium, stir in sour cream, a tablespoon at a time, then add remaining 1/2 teaspoon sugar and reduce heat to low. Cover and simmer 2 - 3 minutes or until sauce is heated through. Taste and adjust seasoning. To serve, place on heated serving platter, and top with straw potatoes. Straw potatoes are very thinly sliced French fries, fried crisp.