Wipe the kombu lightly with a damp cloth and put it into a 2- to 3-quart pot. Add 3 cups water and bring to a boil. When the water begins to boil rapidly, remove the kombu with a pair of tongs (save it for Kombu Relish or for making a second stock) and turn off the heat.

Rinse the mushrooms quickly and throw them into the hot liquid. Let them soak for 30 minutes.

Remove the mushrooms (they can be used in soups and other dishes).

Add the soy sauce and mirin.

Strain the stock through three layers of cheesecloth if the mushrooms have left any grit at the bottom of your pot.