Boil the peeled potatoes in salted water. Take them out into a bowl using a slotted spoon with a little of the hot broth and mash them.
Add 1/2 the butter and pinch of black pepper. Gradually pour in hot milk, while stirring nonstop, until you obtain the desired consistency.
The mashed potatoes need to be a bit thick. At the bottom of a cake form, place thin slices of bacon so that they cover the walls too. Pour the mashed potatoes on evenly.
Cover with the excess bacon and press slightly. Bake in a preheated 356°F (180 °C) oven about 30-35 min. In the meantime clean the mushrooms with a dry towel; if they're too dirty peel them.
Heat a little oil with the remaining butter. Fry the mushrooms to a golden color and season with black pepper and salt.
Invert the mashed potatoes wrapped in bacon onto a plate. Sprinkle with parsley and serve with the mushrooms!