Kosher salt and freshly ground black pepper
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.
- 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. - 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. - 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