You will not be sorry you took the time to make your own stock. As you cook and are busy prepping vegetables and such, e.g., carrots, celery, onions, mushrooms, parsley, and thyme, rather than toss or compost the carrot tops and peels, celery ends and leaves, onion ends and cores, shiitake and button stems, thyme and parsley stems, and any other produce bits you collect, save them. (Skip vegetables like kale, cabbage, broccoli, or anything with a dominating flavor or color that you wouldn’t want in a mushroom stock — no beets!) — Shroom by Becky Selengut
Want to make vegetable stock? Do the same thing, but just use fewer mushrooms and more vegetables (and a big flavor bonus if you roast some of the vegetables as you would the shiitake and onion). If you want to make mushroom stock but don’t have a full bag of trimmings in the freezer, just use an assortment of vegetables and mushrooms (equaling roughly 1 quart) and follow the same general procedure. See the video on making shroom stock at www.shroomthecookbook.com.
Recipes and instructions from Shroom: Mind-bendingly Good Recipes for Cultivated and Wild Mushrooms by Becky Selengut/Andrews McMeel Publishing, LLC.
Preheat the oven to 400 degrees F.
Drizzle a little high-heat oil on a rimmed baking pan. Throw the shiitakes, along with the chopped-up onion, onto the pan, and toss with the oil. Roast until caramelized, about 20 minutes. Deglaze the pan with a little wine or water, scraping up any brown bits stuck to the pan. Dump the mushrooms and onions, along with the liquid, into a stockpot along with the contents of that freezer bag (no need to thaw) and a few rehydrated pieces of dried porcini (along with the strained soaking liquid). Cover with 3 quarts water, chuck in about 5 peppercorns, bring to a boil, lower to a simmer, and cook, uncovered, for 45 minutes.
Pour the contents of the pot through a fine-mesh strainer set over a large bowl, pressing on the solids to extract as much liquid as possible. You should end up with about 2 quarts mushroom stock.