How Many Mushrooms to Use
Use about three ounces of dried mushrooms for every pound of fresh mushrooms called for in the recipe. That might not seem like enough, but once they’re reconstituted, the mushrooms will expand to give you the pound of fresh mushrooms needed in the recipe.
How to Reconstitute Dried Porcini Mushrooms
Dried mushrooms need to be reconstituted before they’re used in place of fresh mushrooms. Reconstitution is just a fancy word for rehydration, and there isn't much to it. Just cover the mushrooms with enough room temperature water to cover them completely and allow them to soak for at least 30 minutes on the countertop. Once they’re finished soaking, remove the mushrooms, reserving the liquid, and rinse them in cool water to get rid of any residual grit. Then add them to your recipe and proceed as written.
The soaking liquid, now transformed into a mushroom broth, is quite flavorful and well worth saving. Strain it through a coffee filter or a paper towel to remove the grit. You can then use it in place of some of the liquid in your recipe to add a bit more mushroom flavor or freeze it to use later in soup, risotto, and more.