Bring a large pot of lightly salted water to a full boil. Fill a bowl with ice water. Blanch the spinach in the boiling water for 20 to 30 seconds, then use tongs to transfer the spinach to the ice water. Drain and squeeze excess water from the spinach. Chop coarsely.
Next, make the dressing. Toast the sesame seeds in a small skillet over medium heat, shaking the pan constantly until seeds turn light golden brown and have a "nutty" fragrance. Transfer 2 tablespoons of the sesame seeds to a mortar and pestle or spice mill, reserving the remainder for garnish. Allow it to cool, and then grind the seeds just until crushed.
Combine the soy sauce and sugar (or maple syrup in a small bowl, and stir until the sugar has dissolved. Add the rice vinegar (or mirin), then stir in the sesame seeds. Toss the dressing with the spinach until it's lightly coated with the dressing.
To serve, drizzle the sesame oil and reserved sesame seeds over the spinach, and serve immediately.