If needed, trim the very ends from the spinach leaves. Prepare a bow with iced water. Place the spinach in a pot or bowl of boiling water, adding the stems first to give them slightly longer to cook then adding the leaves. Stir so the leaves wilt down, then once wilted transfer to the iced water. Leave a minute to cool completely.
Lightly toast the sesame seeds in a dry skillet/frying pan over a medium-low heat. If they are untoasted, toast until gently golden all over, tossing now and then to turn. If already toasted, just warm briefly to get them slightly nutty. Watch closely as they can burn quickly.
Transfer the toasted sesame seeds to a mortar and grind with a pestle until a coarse sandy texture - they don't need to be really fine, a few whole seeds is fine. Alternatively, use a spice grinder or food processor but take care not to grind too much.
Add the sugar and soy sauce to the ground seeds and mix well to form a paste.
Drain the spinach and squeeze well to remove excess water. Cut the spinach roughly into 1 inch/2.5cm lengths or slightly larger. Place the spinach in a bowl and add the sesame paste (or add the spinach in with the paste if your mortar is big enough. Mix well so the spinach is evenly coated - you want to break up clumps of the sesame as well as clumped together spinach. Serve or store in the fridge until needed.