Mix all the ingredients for the miso sauce in a saucepan over low heat. Turn off heat when sugar is dissolved. Set aside.
Rinse spinach leaves well and trim off crowns.
Boil a pot of water and add salt and sugar. Add spinach and parboil for about 1 minute. Remove and rinse in cold water. Drain and gently squeeze to remove any excess water. Portion out into 4 bundles.