Place 2-1/2 tbsp sesame seeds in blender with remaining dressing ingredients; blend until smooth.
Bring a large pot of water to a boil. Add spinach; cook 1 minute. Drain, rinse under cold running water; drain again. Lay 2 or 3 bunches spinach together on a work surface so they form a bundle about 1 inch in diameter; squeeze to remove as much water as possible. Trim and discard root ends; cut each bundle into 2-inch lengths. Repeat with remaining spinach.
Arrange bundles on a serving platter; pour dressing on top. Sprinkle with remaining sesame seeds.