Set a large pot of water on the stove and bring it to a boil.
Wash the spinach thoroughly in a large bowl of water. Pay particular attention to where the stems meet the roots as these crevices tend to trap a lot of grit.
Grind the sesame seeds using a mortar and pestle, clean coffee grinder, or food processor. It should resemble damp sand.
Add the sugar and soy sauce to the ground sesame seeds and stir to combine.
Add the spinach to the boiling water with the roots and stems going in first. Once you have the spinach fully submerged, Bring the water back to a rolling boil.
When the water is boiling, drain the spinach and rinse with several changes of cold water to cool the spinach down rapidly.
Pick the spinach out of the water with the root ends so the leaves all lineup. Fold the spinach in half once and squeeze any excess water out. You don't want the spinach to be soggy, but it should not be completely dry.
Spread the squeezed spinach out on a cutting board and cut it into 1 1/2-inch lengths.
Toss the spinach together with the sesame seeds until they're evenly distributed. Sprinkle with additional sesame seeds and serve.