Cover the boletus with lukewarm water and soak for about 30 minutes. Sweat the diced onion in olive oil until translucent. Then add the porcini mushrooms with the soaking water and simmer until the liquid has almost disappeared.
Pour a glass of game stock (approx. 450 ml) and reduce the liquid by a third. Thicken the sauce with the flour butter, then remove it from the hotplate immediately.
Season with salt, pepper and nutmeg to taste, add the chopped parsley. At the end stir in the crème fraîche.
Serve sprinkled with freshly grated parmesan on ribbon noodles.