In a bowl, place dried ceps, cover with milk and leave them for at least 1 hour to rehydrate. The milk will get a brown color.
COOK POTATOES
Peel potatoes, cut them into large cubes. Cook potatoes: either in a steamer for 12 to 15 minutes depending on the size or in a large volume of salted water.
Check if potatoes are cooked using a sharp knife, then drain.
Sauté and mix ceps
Heat a knob of butter in a pan. Drain mushrooms but do not throw away the milk that will be used later.
Sauté ceps for a few minutes. Add the drained milk, fry for a few more minutes to reduce a bit.
Mix this preparation to get a smooth mushroom purée.
PREPARE purée
Place the drained potatoes in a large bowl. Mash them using a food mill or a potato masher adding the mixed mushrooms, cream and a knob of butter.
Season, taste and add more butter, cream and/or milk depending on your taste and the texture you wish.