Bring a large pot of salted water to a boil. Fill a large bowl with ice water. When the water comes to a boil, dunk the spinach into the pot. Cook for 30 seconds, then drain and immediately plunge into the ice water bath. Let sit for 3 minutes, swishing it around, until cool.
While the spinach is cooling, make the sauce. Place the sesame seeds in a small, dry skillet. Heat over medium heat until you can smell the sesame and they start to turn golden brown, about 3 minutes. As soon as the seeds start to color and the aroma becomes notable, remove them from the heat — they can go from light brown to overtoasted very quickly.
Turn the sesame seeds out of the pan into a mortar or a small food processor. Grind or pulse to crush the seeds. Add the soy sauce, sake, mirin, and sugar, and use a mortar and pestle or the back of a fork to mush together.
Drain the spinach and use your hands to squeeze out all of the excess water. Give the spinach a few chops with a knife on a cutting board. Toss the spinach with the dressing and mound onto a plate.
Serve cold, topped with additional sesame seeds and bonito flakes, if desired.
Notes
You can double the mirin instead of using the sugar if you prefer.
You can make gomae a few days in advance and store in an airtight container in the refrigerator.
Leave out the bonito flakes for a vegetarian/vegan dish.
Use tamari or gluten-free soy sauce if there are gluten intolerances.