Bring a pot or large pan of water to boil, with a little salt added. Add spinach to boiling water and cook for a minute. Take the spinach out as soon as it starts to wilt a little (don’t wait until it’s completely wilted). Shock it in a bowl of cold water and ice (this stops it from cooking).
Remove the spinach from ice water and squeeze out as much water as you can without pressing too hard on the spinach. You’ll end up with a ball about the size of your hand. Roughly chop it, squeeze out a bit more water and gently separate the leaves.
Put the spinach in a bowl and add soy, mirin, sesame seeds, and sesame oil. Mix well and serve.
Notes
You can keep this oshitashi recipe in the fridge for 3-5 days.