This soup is super creamy in texture, but it contains no actual cream. The butter and flour mixture -- what's called a roux -- gives it body and silkiness.
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.
To store soup, let cool, then cover and refrigerate, up to 1 day.