Dive the spinach leaves for 30 seconds in salted boiling water, and immediately transfer them to a bowl of cold water and ice (this way they keep their bright green colour).
Lay them on a towel to drain out the water.
In a pan, roast the sesame seeds for 30 seconds.
Add the soy sauce and sugar to the pan, and reduce on medium heat, stirring frequently, until you it becomes like caramel.
When cold, add the sesame-caramel sauce to the spinach, stir well and serve chilled.