In a saucepan heat the dashi and whisk in the miso pastes. Bring to a simmer and add the tofu, scallions, and mirin, if using. Remove from the heat and serve immediately.
In a saucepan, combine the water and kombu. Bring to a simmer, uncovered, over medium heat. Remove the kombu and discard. Bring the liquid to a boil, add the bonito flakes and immediately remove the pan from the heat. Allow the dashi to sit undisturbed for 2 minutes. Strain through a fine mesh strainer into a medium bowl. Discard the bonito flakes. Use immediately or store, covered, in the refrigerator for up to 3 days.
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