Bring an 8-quart pot of salted water to a boil. Fill a medium bowl with ice water. Add the spinach to the pot, pressing it into the water with a slotted spoon, and cook for 20 seconds. Remove with the spoon and plunge into the ice water. Transfer the spinach to a colander, spreading it out to drain. Using a clean, folded dishtowel, press down on the spinach to remove as much water as possible. Toss the spinach, spread it out, and press it again if necessary, using a fresh towel. Line a baking sheet with paper towels, spread the spinach on top, cover with paper towels, and chill for 30 minutes
Using a spice grinder, grind 2 Tbs. of the sesame seeds with 1/2 tsp. of the sugar. In a medium bowl, combine the remaining 1/2 tsp. sugar with the soy sauce, vinegar, and sesame oil. Stir in the ground sesame seeds.
Add the chilled spinach and gently toss it with your hands, separating any clumps. Serve sprinkled with the remaining sesame seeds.