The potatoes are washed, peeled and cut into cubes. Boil them in water and season them with a broth cube. When boiled, remove from the heat and drain them.
Mash and dilute them with the water in which they were boiled and pour in the milk. Add the pepper and butter, and salt if needed.
In a pan, fry the mushrooms with a little butter. Add them to the soup and serve with croutons.