You may be used to eating spinach leaves in salads, or sautéed. In Japan spinach is rarely eaten raw. The most common way to eat spinach is to blanch it briefly. You may lose some nutrients when you do this, but it’s more than made up for I think by the fact that you can eat a whole lot more spinach than in a salad or so.
In the U.S. and Europe, it’s probably easier these days to buy ready-washed bags of the leaves only. This is a bit of a shame really, because spinach stalks and roots have a different texture which adds interest. In any case, the instructions here assume that you are dealing with the leaves only.
Wash the leaves just to be sure they are totally clean.
Bring a pot of water to the boil. (If you are in a hurry, boil the water in an electric water kettle, then pour the water into the pot.)
Put the spinach leaves in the pot all at once. If you have baby leaves (they are round and small and not crinkled), boil them for 30 seconds and not any longer. If you have fully grown leaves, boil them for about a minute.
Immediately drain the pot. Run cold water over the leaves to cool them off fast. Drain.
Take the spinach leaves in your hands and squeeze out the water as much as you can. You’ll end up with one or more ‘logs’ of spinach looking like this:
This ‘log’ started out life as a whole 200 g (about 7-8 ounces) bag of baby spinach leaves! Cut the ‘log’ into pieces that are a bit shorter than the height of your bento box.
The easiest way to flavor blanched spinach is to just sprinkle some soy sauce on it. You can garnish it with a little bonito flake, sesame seeds, red pepper flakes, etc. I prefer to sprinkle the soy sauce just before eating, so I put it in a small soy sauce bottle. (There’s no need to fill the bottle up incidentally, since you’ll only need a few drops!)
You can vary the flavor by using commercial mentsuyu (soba noodle sauce). For homemade versions, see kaeshi  and Japanese essence . (For what it’s worth, I make kaeshi more nowadays than Japanese essence.)
This is a little more work to make, but is very delicious, and great for bento! This amount makes enough for 200g / 1/2 pound of raw spinach, cooked.
If you don’t have pre-toasted sesame seeds (irigoma), toast the seeds briefly in a small dry frying pan. Crush in a small mortar and pestle or suribachi. Add the sugar and crush some more. Add the mirin and soy sauce and mix. Mix with the spinach.
You can make a sort of cheater’s gomaae by using about 2 teaspoons of tahini instead of the sesame seeds. Add some whole sesame seeds on top for garnish.