For the potato gratin with porcini mushrooms, bring the soup to a boil and pour over the mushrooms. Soak for about twenty minutes.
In the meantime, peel the potatoes and score them quite deeply on the top, making sure that the tubers still hold together. Place them close together in a buttered baking dish.
Layer the soaked porcini mushrooms in the spaces between the potatoes. Add the remaining soaking water. Pour the cream over the gratin. Season everything with salt and pepper.
Bake the potato gratin with porcini mushrooms in the oven heated to 200 degrees on the lowest shelf for forty to fifty minutes.