Beef Stroganoff

1 tbsp powdered mustard
1 tbsp sugar
2 tsp salt
4-5 tbsp vegetable oil
4 cups thinly sliced onions separated into rings
1 lb fresh mushrooms, thinly sliced lengthwise
2 lb fillet of beef, trimmed of all fat
1 tsp freshly ground black pepper
1 pint sour cream

1. In a small bowl combine the mustard, 1 ½ tsp of the sugar, a pinch of the salt and enough hot water (perhaps a tbsp) to form a thick paste. Let the mustard rest at room temperature for about 15 minutes.
2. Heat 2 tbsp 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, simmer 20 to 30 minutes or until the vegetables are soft. Drain them in a sieve, discard the liquid and return the mixture to the skillet.
3. With a large, sharp knife cut the fillet across the grain into ¼-inch-wide rounds. Lay each round on a board and slice it with the grain into ¼-inch-wide strips. Heat 2 tbsp 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, 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 additional oil if necessary. When all the meat has been combined with the vegetables, stir in the remaining salt, pepper and the mustard paste. Stir in the sour cream, a tbsp at a time, then add the remaining ½ tsp 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.
4. Serve with frites.