Put the water, light soy sauce, and mirin in a pot and bring to a boil.
Stop the heat, and after it cools a bit, add dashi stock and let it cool.
Wash the spinach in plenty of running water.
Prepare cold water in a large bowl (In the summer you'll want ice water, but in the winter water is OK).
Add salt to plenty of water, then once it's come to a boil, add spinach and boil.
Put the spinach in cold water you prepared in Step 4.
Once the spinach has cooled, wring it dry, then cut into 5 cm.
After draining thoroughly again, soak in the flavoring liquid from Step 1 for flavor.
Transfer to a serving dish and top with bonito flakes. Pour soy sauce over while tasting.
Story Behind this Recipe
There are times when I only use soy sauce and bonito flakes, but sometimes I make it without cutting corners. Mizuna, komatsuna, and Chinese cabbage are also delicious. You can also boil and add enoki or shimeji mushrooms. They are tasty as well.
If you heat the dashi stock, it loses its smell, so avoid adding it to Step 1. It looks delicious if you mound it generously.