Put the cleaned and sliced mushrooms to boil until soft in about 3 cups (700 ml) of water, then remove from the broth with a slotted spoon.
Season the broth to taste with salt and pepper. Preheat oil in another dish and fry the flour in it, while stirring with a whisk. While still stirring continuously, add the milk and dilute the broth until you get the desired consistency.
Add mushrooms and once the sauce begins to bubble, take it off the heat. Serve as a side dish to meat and vegetable dishes.