Place 1 1/2 tablespoons sesame seeds on a plate; use the bottom of a pot or heavy measuring cup to crush the seeds. Transfer to a bowl. Add water, soy sauce, and sugar; stir dressing.
Bring a small pot of water to a boil; add salt. Add spinach and cook until wilted, no more than 1 minute. Drain spinach into a colander and rinse with cool water to stop the cooking process. Squeeze spinach with hands to remove moisture. Cut into strips.
Place cooked spinach in serving bowls. Drizzle dressing on top and add remaining sesame seeds as garnish.