The stems of shiitake mushrooms are too fibrous to eat, but they still pack a lot of flavor. Just a few stems can infuse a broth with rich flavors and earthy aromas. The stems are especially good in vegetable and broth-based soups (as opposed to thick stews).
Even cooked this way, the stems still aren't good to eat so be sure to discard them (for real, this time!) before serving. If you won't be straining the broth, wrap the mushrooms stems in cheesecloth like a bouquet garni so you can easily pick them out.
Mushroom stems can also be frozen indefinitely. We keep a container of them in our freezer, adding stems when we use the tops and taking some out when we make soup.
Give it a try!