Peel the potatoes and cut them into pieces, not too large though, in order to be able to mash them. Boil the potatoes and carrots. Shortly before being fully boiled, add the finely chopped mushrooms.
Once the mushrooms are boiled, remove them from the heat and blend and dilute them with the broth in which you cooked the vegetables. Put them back on the stove and reduce the heat to medium, stirring frequently.
It can also be eaten cold. You can use it as a sauce for roast meat as well.