Blanch spinach: Bring a pot of water to a boil. Add spinach and cook for 30 seconds. Immediately add to a bowl of ice water, or drain and rinse under cold water to prevent further cooking.
Remove excess water: Once the spinach has cooled, gather spinach in handfuls and squeeze out excess water.
Chop: Roughly chop the spinach into 1-inch segments and place in a bowl.
Grind sesame seeds: Grind the toasted sesame seeds. (Typically with a pestle and mortar, but you can try using a coffee/spice grinder, food processor, or blender. You may need to double or triple the quantity.) It doesn't need to be finely ground, some whole seeds are fine.
Make sauce: Add sugar and soy sauce (or tamari/coconut aminos) and mix in with the ground sesame seeds.
Thin sauce: Add water as needed to make the sauce easier to mix with the spinach.
Toss: Add sesame sauce to spinach and mix well until spinach is evenly coated.
Notes
Soy-Free/Whole30 Option: Replace sugar and soy sauce with 1 tbsp coconut aminos*.