For the miso soup:
2 cups dashi
4 ounces (1/4 block) silken or firm tofu
1 to 2 scallions
2 tablespoons white miso paste
Make the dashi. Prepare the tofu and scallions: Cut the tofu into very small cubes, 1/4-inch to 1/2-inch on each side. Slice the scallions very thinly.
Bring the broth to a rapid simmer: Pour the dashi or broth back into the saucepan and bring to a rapid simmer over medium-high heat.
Mix the miso with 1/2 cup hot broth: Place the miso in a small ramekin or measuring cup. Scoop out about 1/2 cup of the broth and pour it over the miso. Whisk with a dinner fork or whisk until the miso is entirely dissolved in the water and no lumps remain.
Pour the miso into the broth: Pour the dissolved miso into the simmering broth.
Add the tofu: Reduce the heat to medium-low and add the tofu to the miso. Simmer just enough to warm the tofu, 1 to 2 minutes. Do not boil the miso once the tofu has been added.
Add the scallions: Just before serving, scatter the scallions over the top of the soup.
Serve in individual bowls: Pour the miso into individual bowls and serve. Miso is best when served fresh. It will settle a bit as it sits in the broth; whisk briefly with chopsticks or a spoon to mix the soup again.