The key to great mushroom soup is to cook the mushrooms in butter long enough to drive off excess moisture and allow them to start to brown, deepening their flavor.
NOTE: You can use plain button mushrooms, but for the best flavor, use a mix of mushrooms, such as buttom, portobello, shiitake, and/or other foraged or cultivated mushrooms.
2 pounds mushrooms (see Note above), cleaned and sliced ¼ inch thick (about 3 quarts)
4 tablespoons unsalted butter
1 large leek, white and pale green parts only, split in half and cut into ¼-inch-thick half-moons (about 1 cup)
1 medium onion, finely sliced (about 1 cup)
2 teaspoons fresh thyme leaves
3 tablespoons all-purpose flour
1 cup milk
4 cups homemade or low-sodium canned chicken stock, plus more if necessary
2 bay leaves
Kosher salt and freshly ground black pepper
1. Set aside 1 cup of the mushrooms. Melt 3 tablespoons of the butter in a large Dutch oven or soup pot over medium-high heat. Add the remaining mushrooms and cook, stirring occasionally, until they have given off their liquid and are beginning to brown, about 10 minutes. Add the leeks, onions, and half of the thyme and cook, stirring frequently, until the vegetables are softened, about 5 minutes.
2. Add the flour and cook, stirring constantly, until all the flour is absorbed, about 30 seconds. Stirring constantly, slowly pour in the milk, followed by the stock. Add the bay leaves and bring to a boil, then reduce the heat to maintain a simmer, cover, and cook, stirring occasionally, until the liquid is thickened and lightly reduced, about 10 minutes. Discard the bay leaves.
3. Working in batches, transfer the mixture to a blender and blend, starting on low speed and gradually increasing to high, until a rough puree forms, about 1 minute; add additional stock or water if necessary to thin to the desired consistency (I like mine thick). Pass through a fine-mesh strainer into a clean pot and season to taste with salt and pepper. (Alternatively, use an immersion blender to blend the soup directly in the original pot.) Keep hot.
4. Melt the remaining tablespoon of butter in a large nonstick skillet over medium-high heat. When the foaming subsides, add the reserved 1 cup mushrooms and cook, stirring and tossing frequently, until deep brown, about 8 minutes. Add the remaining thyme and season to taste with salt and pepper.
5. Serve the soup garnished with the sautéed mushrooms.