Before I tried this soup, mushroom soups to me were always pureed soups. I don't recall seeing mushroom soup anywhere in the form of soup, that is, in a more liquid state, not ground in a blender. And if I have, they were apparently unremarkable, since I don't remember it. Mushroom soups always have one big drawback: the mushrooms. They soak up, give their flavor to the broth, and remain virtually tasteless themselves. You end up eating a soup that seems good, but you're haunted by a watery feeling. The good news is that you won't feel that way in this recipe. The bad news is that you can say goodbye to mushroom soups. The recipe is a bit fiddly (just a little) and takes a while to prepare, and you probably won't eat other mushroom soups after this soup.After the soup is ready, it is beaten with a blender and passed through a sieve. The watery and tasteless mushrooms are separated, as well as the rest of the vegetables that gave their taste, and only a rich and flavorful broth remains. Then the broth is whipped with a blender again until it forms an airy foam. This little trick of serving very much affects the taste and feel of the dish, do not neglect.Another trick to this soup is the mushroom powder that is sprinkled on the soup just before serving. It makes the flavor of the soup more mushroomy, infuses the flavor with umami, also looks spectacular.
Preheat the oven to 150 °C / 300 °F.Roast the shiitake and porcini mushrooms for 5 minutes. Remove from the oven and cool. Grind them with a blender, food processor or coffee grinder. Sieve them through a fine sieve. Store in a tightly closed container.
Slice the mushrooms, onions, leeks and potatoes thinly.
In a wide saucepan, melt the butter, add the onions and potatoes and stew over low heat, stirring, for about 10 minutes.
Add leeks and mushrooms and cook stirring for about 15 minutes more, until soft.In the meantime, heat 1.5 liters of vegetable broth.
Increase the heat to high and pour in the dry vermouth. Boil out almost all the liquid, stirring frequently. (1 to 2 minutes). Pour in vegetable broth, add cream and milk, bring to a boil, reduce heat to low. Cook for 10 minutes.
Remove from the heat and puree until smooth. Strain the soup through a fine sieve, discard the cake. Salt and pepper to taste.
Whisk the soup with a blender until lightly foamy. Pour into plates and serve. Be sure to sprinkle each portion with mushroom powder before eating.