Wash the mushrooms thoroughly, pour cold water over them, and keep that way for 5— 8 hours. Transfer into some other vessel and pour the same liquid (preliminarily strained through a piece of cheese-cloth) over them. Cook on low heat until soft. Wash the cooked mushrooms with hot water and chop fine. Make a flour dressing from 2 or 3 tablespoons of flour and 1 spoon of butter. Separately heat the finely chopped onion until light yellow.
Slightly chill and dilute the flour dressing with the mushroom liquid. Boil, stirring and skimming the scum. Put the chopped mushrooms and onion in the sauce, salt, pepper, and boil for another 5—10 minutes.
You can add sour cream to the sauce, in which case the mushrooms are cooked in a lesser amount of water.