Bunch together the stems of the spinach and cut off their roots. Blanch in hot salted water, then immerse in cold water. Drain thoroughly.
Add the dashi, soy sauce, salt and mirin into a pan, and when they reach boiling point, dip the spinach leaves into the pan, remove immediately and cool. Allow the dashi in the pan to cool. When cold, place the spinach into the dashi and leave to soak for 2-3 hours.
Remove the spinach from the pan and cut into bite-size pieces, then place in a bowl and sprinkle on the bonito flakes.