A critical building block of vegetarian Japanese cooking, this stock is full of naturally-occurring flavor enhancers from kombu seaweed and shiitake mushrooms. Ideally, the seaweed and mushrooms will soak for many hours, but that isn't essential. A decent stock can be made in a hurry.
Place a piece of kombu sea vegetable and several dried shiitake mushrooms into the water. For best results, use a glass or ceramic container.
Let this mixture steep for a few hours or overnight in the refrigerator. A long soaking allows the natural glutamates (flavor enhancers) to develop and dissolve into the water.
Save the shiitake mushrooms for another use (recipes that call for dashi also often use shiitake mushrooms). The kombu is often discarded, though there are certainly recipes that use it (see notes).
Kombu is available in many Asian grocery stores (generalists like 99 Ranch Market, and specialists in Japanese or Korean foods). Sometimes packages simply read "Kombu", sometimes they say "Dashi Kombu."
The kombu is often discarded, though there are certainly recipes that use it: Washoku has a one for a flavorful kombu relish; the team at Salt Point Seaweed recommend putting pieces in a pot of beans for extra umami (store the pieces of kombu in your freezer).
Adapted from Washoku: Recipes from the Japanese Home Kitchen, by Elizabeth Andoh