Clean mushrooms with a dry or damp clean kitchen towel or paper towel. Slice the mushrooms and mince the garlic.
Add the olive oil, mushrooms, and a pinch of salt and pepper to a soup pot. Sauté the mushrooms over medium heat until they have released all of their moisture, all the liquid has evaporated from the bottom of the pot, and the mushrooms become very dark brown. Add the minced garlic and continue to sauté for one minute more.
Add the butter and flour to the pot. Stir and cook for about two minutes more, or until the butter coats the bottom of the pot and turns deep golden brown.
Once the flour mixture is a bit browned, add the vegetable broth, soy sauce, and thyme. Stir to dissolve all of the flour from the bottom of the pot. Allow the liquid to come up to a simmer, at which point the flour will slightly thicken the broth.
Finally, stir the cream into the soup. Give the soup a taste and adjust the salt or pepper to your liking. Serve with crusty bread and enjoy!
*I highly suggest using baby bella mushrooms (aka cremini mushrooms) for this soup because they have a nice deep color and rich flavor. White button mushrooms will not add as much color or flavor to the soup. You could also chop up some larger portobello mushroom caps to use in place of the baby bellas. Some readers have also added dried shiitake mushrooms to this recipe (soak them in hot water first to rehydrate).**Vegetable broth is the base for this soup, and it provides complexity to the flavor, as well as a nice brown color. You can use mushroom broth instead if you have it. I use Better Than Bouillon to make my broths.***Heavy cream adds a deliciously rich and creamy flavor to the soup. I've only made this soup with heavy cream, but some readers have successfully swapped the cream for half-and-half or evaporated milk!