Bring dashi to a boil. Pour the dashi into a medium saucepan and bring it to a gentle boil over medium heat.
Add wakame and green onions. When the dashi starts boiling, add the wakame and green onions. (Reserve a little bit of the green onions for garnish). Reduce the heat to low and keep the soup at a simmer.
Add miso. Add the miso paste into a small bowl together with a little bit of dashi. Dissolve the miso completely before adding it to the soup. Do not let the soup boil.
Add tofu. Finally, add the tofu. If you add the tofu before the miso is dissolved, you might break it.
Serve. Divide the soup between individual bowls. Garnish with the reserved green onions and serve immediately. The miso will settle a bit as it sits in the dashi; whisk briefly with chopsticks or a spoon to mix the soup again.
Store. Leftover miso soup keeps well in the refrigerator for 2-3 days.