Heat 1 cup of water and the dashi packet or kombu in a small saucepan. Stop right before the water boils, allow to steep off of the heat for a few minutes, then strain. You can reserve the solids and re-use them (I like to slice the kombu thin and add it to salads or pickles, dashi packets can go in with veggies and scraps next time you make stock).
Add soy sauce and mirin (optional) until you have a lightly salty and rich tasting broth. If you're using Yagenbori as a garnish add a bit less soy sauce as the Yagenbori has some salt.
Prepare a bowl of ice water and heat a large pot of lightly salted water. Rinse your spinach thoroughly to remove any sand or dirt.
Blanch the spinach, lower the bunches in stem-end first, allow to cook for about twenty seconds, then use a wooden spoon to push the leaves under. The spinach is fully blanched when bright green and tender-crisp (about another 45 seconds of cooking).
Remove to the ice water and mix well to cool the spinach rapidly. Then squeeze any excess water out with your hands. Chop the spinach into 2-3 inch long pieces, and add them to a container with a cover.
Pour the broth over the spinach and put it in the refrigerator. The flavor peaks after a few hours, and it will keep, covered, for 3 days, so this is a great way to use up spinach and give it a bit more life.
To serve, remove however much spinach you'd like to eat to a bowl, top with a bit of the broth, then garnish with toasted sesame seeds and/or Yagenbori
Variations & Ideas:
• It won't be as deeply flavorful, but you can always skip the dashi and just add a good soy sauce to blanched spinach. I do this occasionally when I forget to make the dish ahead of time. A little bit of shaved katsuobushi on top adds great flavor if you go this route.
• You can use the same prep for plenty of other vegetables as well, try thin slices of eggplant, okra pods, escarole, or Swiss chard.
• If you have leftovers they are great chopped a bit more finely and added into an omelette or frittata.
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