Finely dice the onion and potatoes. Heat the oil in a saucepan and sweat both in it until translucent. Pour in the vegetable stock and cover and let the soup simmer for about 15 minutes.
Cut the mushrooms into slices. Remove half of the soup, puree and return to the pot. Add the mushrooms to the soup and cook for about 5 minutes. Season the soup with salt, pepper and nutmeg before serving.