Step 1 Combine the kombu and water in a saucepan and bring to a simmer over medium heat. Remove the kombu and bring the liquid to a simmer. Remove from heat. Add the shiitake mushrooms and trimmings and simmer very gently for 5 minutes. Remove from heat and strain the solids, discarding them. This makes about 1 quart dashi, more than is needed for the remainder of the recipe. The dashi will keep, covered and refrigerated, up to 5 days.