This is a light soup, for a first course or light lunch.
If you want to use it as a sauce simply add less of the stock when you purée it.
Truffle oil is nice, but a good olive oil or walnut oil would be good, too. Or skip it.
A variety of mushrooms will give the most flavor.
Keywords: mushroom soup, first course soup
Find it online: https://thymeforcookingblog.com/2019/11/cream-of-mushroom-soup/