Boil the spinach: Bring a pot or large pan of water to boil, with a little salt added. Add spinach to boiling water and cook for 1 minute. Take the spinach out as soon as it starts to wilt a little – don’t wait until it’s completely wilted – and shock it in a bowl of cold water filled with ice.
Squeeze and chop: Take the spinach out of the water and squeeze out as much water as you can without pressing too hard on the spinach. Roughly chop it, squeeze out a bit more water and gently separate the leaves.
Mix the spinach with the seasonings: Put the spinach in a bowl and add soy, mirin, sesame seeds, and sesame oil. Mix well and serve.
Notes
Keep oshitashi in a storage container and refrigerate for up to 4 days.