Bring a large pot of water to a boil. Add the spinach, and cook only until it is bright green, and wilted, up to 30 seconds (note that it is best to do this in 2 batches to prevent the spinach you put in the pot first from becoming overcooked). Immediately transfer the spinach to a large colander and rinse with cold water until it is cooled completely. Leave it to drain while you prepare the dressing.
Add 2 Tablespoons sesame seeds to a clean coffee grinder, or mini food processor and process until the sesame seeds are coarsely ground. Add the ground sesame seeds, 2 Tablespoons tahini, 1 Tablespoons rice vinegar, 2 Tablespoons reduced-sodium soy sauce, 1 teaspoon mirin, and 1 teaspoon granulated sugar to a medium bowl and whisk to combine. It should become quite thick, almost like mayonnaise. Reserve a couple of spoonfuls of dressing in a small bowl if you wish to drizzle any dressing on top of the spinach (as seen in the photos).
Gently squeeze any excess water from the spinach, and pat dry with a clean dishtowel or paper towel. Add the spinach to the bowl with the dressing and toss to coat. If you like, transfer the spinach to a serving dish, or divide the spinach into 4 small bowls to serve. Drizzle with reserved dressing and sprinkle with additional sesame seeds. Serve immediately.