Miso soup is a key part of a traditional Japanese breakfast. At its simplest, miso soup is made of nothing more than dashi (“broth” in Japanese) and miso, but the variations and adaptations are endless. The version below is designed to highlight peak-season summer squash. The squash provides textual contrast to the soup and a pop of color, whether you choose to use zucchini, pattypan or yellow squash. When seasoning miso soup it’s a good idea to taste as you go. Not all miso has the same saltiness, so you may not need the full amount suggested in the recipe, or you may choose to add a bit extra. Trust your palette.
Soak the wakame in 2 cups water for 10 minutes. Drain and set aside.
To make the dashi, combine the kombu and filtered water in a medium saucepan and soak at room temperature for at least 2 hours and up to 10 hours. After soaking, heat the saucepan with the water and soaked kombu over low heat. Continue to heat until bubbles begin to form around the kombu, 7 to 10 minutes. Remove the kombu before the water comes to a boil. Save the kombu for future use.
Bring the soaking water to a boil and then turn off the heat. Add the bonito flakes. Let stand for 2 minutes without stirring to steep the bonito flakes. Strain the dashi through a fine-mesh sieve or strainer lined with a thin kitchen towel, two layers of cheesecloth or paper towels. Do not press the bonito flakes while straining as it will cloud the dashi. You should have 3 1/2 to 4 cups of dashi. Use the finished dashi immediately, or cool completely and refrigerate up to 1 week.
Return the dashi to the saucepan and bring to a boil over medium-high heat. Add the squash, adjust the heat to maintain a low simmer and continue cooking until the squash reaches the desired tenderness, 3 to 4 minutes. Add the wakame. If the dashi evaporates below 3 cups (710 milliliters), replenish with filtered water or additional dashi to bring it back up to 3 1/2 cups (840 milliliters). Carefully add the tofu and continue simmering just until heated through, about 1 minute.
Meanwhile, place the miso in a small bowl. Ladle in about 1/4 cup (60 milliliters) of the hot dashi and stir until a loose paste is formed. Turn the heat off under the saucepan, add the miso mixture to the soup and mix well. Taste the soup and make adjustments as needed with additional miso or dashi. Divide the soup among bowls, making sure the ingredients are distributed evenly and attractively. Add a dash of shichimi togarashi for heat and garnish with the chives if desired and serve.