Toss the spinach with the sesame sauce just before serving.
Notes
Salting the cooking water helps preserve the spinach's bright green color.
Cutting a small cross into the stems and adding them before the leaves ensures even cooking; transferring the spinach straight to cold water helps set the color.
Shōyu-arai (drizzling the blanched spinach with a little soy sauce, then squeezing it) seasons it while removing excess water, which keeps the sesame sauce from becoming diluted.
As with many Japanese dressed vegetable dishes, toss the spinach with the sauce just before serving to avoid excess moisture.