For the cream of mushroom soup, soak dried porcini mushrooms in cold water for about 15 minutes. Bring water to a boil, add salt, pepper and garlic.
Strain porcini mushrooms and add to soup with diced potatoes. Cook until soft.
Mix sour cream with flour until smooth, then stir into soup. Season to taste. The Rahmschwammerlsuppe should not be boiled again afterwards.