Boil water in a pot, add the komatsuna starting with the stems, blanch briefly, then rinse in cold water. Squeeze out excess water and cut into bite-sized pieces.

Reheat the water in the pot, add the fried tofu, and blanch briefly. Drain and cut into bite-sized pieces.

In a microwave-safe container, combine water and bonito flakes, cover loosely with plastic wrap, and microwave at 600W for 2 minutes. Strain the mixture to make bonito broth.

In a pot, combine the bonito broth, mirin, salt, and soy sauce. Heat over medium heat until boiling, then add the fried tofu and simmer on low heat for about 5 minutes.