The first time I made dashi I was shocked by how easy it was. Just cover kombu with water, and let it sit. That’s it. No chopping, no simmering, no skimming, and usually no straining. Traditionally the liquid is steeped with bonito flakes, but since bonito is made from dried fish, it’s omitted here. Dried shiitake mushrooms add flavor instead. But even without the bonito flakes, dashi is surprisingly rich in umami, in part because kombu contains some of the same naturally occurring glutamates that appear in MSG. I’ve included both cold and hot water methods—the cold water method produces a better-tasting dashi, with a fuller flavor, but when you haven’t been able to plan ahead, the hot water method will work fine. After you get into the habit, you might do as I do and reserve a big Mason jar specifically for dashi.
Combine the kombu, mushrooms, and water in a large container and let stand for at least 30 minutes, or up to 12 hours. It gets stronger as it sits, and the taste can vary depending on what type of kombu you use, so with a few rounds you’ll find your preference. If you plan to let it stand for more than 4 hours, place it in the refrigerator, lidded or covered with a piece of plastic wrap.
Alternatively, bring the water to a bare simmer in a saucepan. Remove from the heat, add the kombu and mushrooms, and let stand for 30 minutes.
Discard the kombu (alternatively, chop it up and use it as a nutritious addition to salads and bowls of rice and other grains). Pick out the mushrooms and trim off and discard the stems. Reserve the mushroom caps for another use. You may want to strain the dashi through a fine-mesh sieve or cheesecloth if there are small pieces of kombu left behind, but I rarely do this.
Stored in an airtight container, the dashi will keep for 2 or 3 days.