Blanch the spinach in boiling water for 15 seconds. Drain and let cool under running water. Squeeze out the water well from the spinach. Trim the roots (if any) and cut the spinach into 2-inch (5 cm) sections.
Combine the mentsuyu, water, and sugar in a bowl, then add the spinach. Mix well.
Chill or serve right away with a sprinkle of katsuobushi.