½cupheavy cream(remove it from the fridge when you start cooking)
2tablespoonscornstarch(optional, but helps to thicken the soup)**
Heat the butter in a large, heavy stockpot over medium-high heat, about 2 minutes. Add the mushrooms, the garlic and the salt and cook, stirring often, 5-7 minutes, until the mushrooms are tender and dark.
Add the chicken broth, black pepper and thyme. Bring to a boil. Reduce the heat to medium-low and simmer, partially covered, for 10 minutes, to allow the flavors to meld.
Turn the heat off. Puree the soup using an immersion blender. Don't try to make it completely smooth - it's nice (and pretty) to leave it a bit chunky with visible mushroom bits. Prior to blending the soup, you can reserve a few mushrooms to use for decoration, as shown in the photos.
Stir the cream into the pureed soup. Taste and decide if you'd like to add more salt - this would depend on how salty your chicken broth is.
Turn the heat back on to medium and bring the soup to a low simmer. Mix the cornstarch with two tablespoons of cold water until smooth and stir the slurry into the soup.
Continue heating the soup (but don't bring it to a boil, lower the heat if needed), stirring, until heated through and thickened, 1-2 minutes. Serve immediately.
*You can use 1 lb. of frozen mushrooms. I like these mixed mushrooms from Whole Foods. They release quite a bit of liquid, more so than fresh mushrooms. But they are very flavorful, and you can use them frozen.**If you'd rather not use cornstarch, you can make a thicker soup by pureeing it completely in a blender rather than using an immersion blender, which isn't as efficient. If you go this route, be careful when blending the hot soup. Do it in batches, and hold the blender's lid down while you blend.