This is an easy cold-brew kombu dashi. It is a versatile vegan dashi that can be used as a base for suimono and misoshiru as well as any dishes that call for dashi or stock. You can also use it like wine—a dash here and there—to bring more flavor to your dishes and soups. The cold-brew method makes a mild dashi, but if you prefer it stronger, cook the soaked kombu in its liquid over low heat for ten minutes. Kombu comes in various sizes, precut or whole. If hydrated, it will double or more in size.
In a large bowl, soak the kombu in the filtered water for a minimum of 3 hours (to a maximum of 10 hours) at room temperature. Remove the kombu and use the liquid immediately, or store it in the refrigerator, where it will keep for up to 1 week. This piece of kombu can be reused one more time to make a secondary dashi (see page 30).