Bring a large pot of water to a boil. Add the spinach, and cook only until it is bright green, and wilted, about 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 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 coffee grinder or mini food processor and process until coarsely ground. Add the ground sesame seeds, tahini, rice vinegar, soy sauce, mirin, and 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.
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.
NOTE: You can add a little reserved spinach water to the dressing to thin it out if desired.
