Heat half the oil in a large, heavy frying pan.
Once the fat is hot, add the sliced onions, reduce the temperature. Saute the onions for 25 min with the lid on, stirring occasionally, until soft. In the last 10 min remove the lid and add the mushrooms. Remove the saute pan from the stove, pour off the liquid and set the mixture aside in a baking dish.
Heat the remaining oil. Pour in half of the meat, stir with a wooden spoon and cook until evenly browned. Using a slotted spoon, add the meat to the vegetables in the baking dish form and saute the remaining meat. When it is also browned, add the remaining meat and vegetables to the skillet repeatedly along with the mustard mixture. Season to taste and add the sour cream, stirring throughout. Saute slowly for about 5 min with the saucepan closed and bring to the table. Sprinkle a tiny bit of parsley over each unit and offer straw potatoes with it, if desired.
Author’s Note: The Stroganoffs became one of the richest merchant families through the ruthless trade in furs from Siberia. A French chef who worked for Count Stroganoff in the late 19th century created this now internationally famous dish. It should *not* be served with long grain rice – a bad habit introduced by the West – but tr