This soup may not be beautiful to look at, but it certainly makes up for it in taste. It starts with a homemade mushroom stock made with shiitake stems—too tough to eat but full of umami to extract—and heaps of fresh tarragon, building depth of flavor and great aroma from the very beginning. A pile of sautéed and simmered shiitakes adds another layer of mushroom flavor, and finishing touches of cream and Parmesan cheese add richness. The drizzle of truffle oil at the end is just an accessory, certainly not a necessity. If you do use it, make sure your truffle oil actually contains slivers of truffle, not just the essence, which tends to be chemically made, sort of like perfume. You can use just one kind of mushroom or a combination of shiitake, portobello, and button. Any way you make it, it is an elegant and warming soup perfect …