Gather all the ingredients.
In a saucepan, put water and kombu.
Slowly bring it to a boil. Once boiling, remove the kombu from the liquid (You can make Kombu Tsukudani with the used kombu). If you are not vegan/vegetarian, you can add a small amount of katsuobushi to make Awase Dashi for more flavors. Please see the instruction here. If you're using dashi powder, please read here.
In Kombu Dashi, add mirin and usukuchi soy sauce.
Mix together and once it boils, turn off the heat.
Rinse the spinach and boil a large pot of water.
When the water is boiling, add the salt and put the spinach in the pot from the stem side first because it takes longer to cook. After 15 seconds, you can push down the leafy part into the water and cook for 1 minute. (US spinach is tender than Japanese spinach, so it’ll cook faster).
Once the spinach is cooked through, pick it up quickly, roughly less than 1 minute.
Transfer to iced water and let it cool (but don’t leave the spinach too long as you will lose the nutrients).
As soon as it’s cooled, collect spinach and squeeze water out. If you like to serve as I do, collect the stem and put together. I like to serve both stem and leafy parts separately instead of mixing it up together.
Cut the spinach into 1 ½ inch lengths and squeeze the water out one more time.
Put the spinach in the airtight container and pour the sauce over.
Make sure the spinach is evenly distributed in the container and soak in the sauce. Put the lid on and let it soak in the refrigerator for at least 1 hour (3-4 hours ideally).
Serve the spinach in a bowl and pour the sauce over. I like to make sure each bowl gets both spinach stem and leafy part. Love the dark and light color contrast to each other.
Sprinkle sesame seeds for vegan/vegetarian version, and bonito flakes for non-vegan/vegetarian version.
You can keep in the refrigerator for up to 3 days.
Recipe by Namiko Chen of Just One Cookbook. All images and content on this site are copyright protected. Please do not use my images without my permission. If you’d like to share this recipe on your site, please re-write the recipe and link to this post as the original source. Thank you.