Ingredients
1/4 cup (1/2 stick) unsalted butter
1 medium yellow onion, diced small
1 1/2 pounds chopped cremini mushrooms
1/4 cup all-purpose flour (spooned and leveled)
4 cups low-sodium chicken broth
Coarse salt and ground pepper
Preparation
In a medium pot, melt butter over medium. Add onion and cook, stirring occasionally, until softened, 8 minutes. Add mushrooms and cook until they soften and release their liquid.
Add flour and cook, stirring, until combined, 1 minute.
Whisking constantly, add broth, 1 cup water, and thyme and bring to a boil over high. Reduce heat and simmer, whisking occasionally, 10 minutes.
Transfer mixture to a large bowl and discard thyme. In batches, fill a blender halfway with mixture and puree until smooth; to allow heat to escape, remove cap from hole in lid, cover lid with a dish towel, and hold down firmly while blending. Return soup to pot as pureed.
Season with salt and pepper. Thin with broth or water, if desired.
