Step 1 In a saucepan, bring the water, soy sauce, mirin and kombu to a boil over medium heat. Boil for 30 to 45 seconds, then remove from heat and add the bonito flakes.
Step 2 Let the broth steep for 3 to 5 minutes. Strain the broth through a fine mesh strainer, discarding the kombu and bonito flakes. Cool the broth, then refrigerate. It can be made up to a week ahead and kept refrigerated and tightly covered. Makes 3 cups.