Boil the washed potatoes in salted water until ready. Peel them and chop into coarse cubes in a tray smeared with a little of the butter.
Heat the rest of the butter in a pan and add the finely chopped leeks, fry until they soften.
To them add the finely chopped mushrooms and fry about 10 min. to a golden color.
Pour in the cream, add salt and pepper to taste. Once it comes to a boil, pour it evenly over the chopped potatoes.
Grate the cheese on top and bake in a preheated 392°F (200 °C) oven for 20 min. until the cheese gains a light crisp.