Step 1 In a medium size pan, bring the water, bonito flakes, softened shiitake mushrooms, mirin and light color soy sauce to a boil over high heat. Reduce the heat and simmer gently for 3 minutes to infuse the flavors. Remove from heat, strain the sauce and discard the bonito flakes. Reserve the shiitake mushrooms; remove any tough stems and slice them thinly and use them as a condiment for the noodles. Let the dipping sauce come to room temperature, then refrigerate, covered until needed. This makes 3 1/2 cups sauce and will keep for 1 week, refrigerated.