Add 2 cups of water and 1 dashi packet to a small pot. Bring to a boil over medium heat, then reduce heat and simmer for 3–5 minutes. Remove the dashi packet and squeeze it gently with chopsticks.
Cut the tofu into small cubes (about 1.5cm). Add the tofu and the dried wakame to the broth and simmer for 1–2 minutes until the wakame rehydrates.
Turn off the heat. Dissolve the miso paste into the broth using a small strainer or by mashing it through chopsticks. Important: do not boil the miso — it kills the flavor and aroma.
Ladle into bowls and top with chopped green onion if desired. Serve immediately.
