Soak kombu for 3 minutes in the 6 cups of water. heat the water with kombu to nearly boiling, but do not boil. Remove and discard kombu. Add bonito flakes, dried matsutake, and turn off heat, allow to infuse for 20 minutes.
Strain, cool and reserve. Discard bonito or save to make furikake.
For the soup
Heat the dashi and miso until simmering.
Add the greens and tofu and heat through.
Double check the seasoning and adjust as needed, then add the matsutake, stir to heat through but don’t try to cook them, turn off the heat and serve.
Video
Notes
For a traditional texture, you can peel the mushrooms apart instead of slicing them which is the traditional way it's done in Japan. Using store bought chicken stock instead of dashi isn't traditional, but it's still very good.