Simmer for about 2 minutes, just long enough to heat the tofu through and gently perfume the broth.
Add the mitsuba, turn off the heat, divide among bowls, and serve right away.
Notes
Clear soups are meant to be delicately seasoned: taste and adjust the salt so the dashi and matsutake remain front and center.Do not overcook the matsutake: its aroma fades quickly, so the brief 2-minute simmer is key to the dish.