Simmer 1 tablespoon of mirin in a small pan for 30 seconds. Remove from the heat.
Coarsely grind 2 tablespoons of toasted white sesame seeds. Mix with the mirin, 2 teaspoons of plain miso paste, 1 teaspoon of sugar, and ½ teaspoon of soy sauce.
Bring a pot of water to a boil with ¼ teaspoon of kosher salt. Hold the stems of 170g of spinach in the water for 15 seconds, then submerge the leaves for 30 seconds.
Rinse under cold water. Squeeze dry, cut into 2in / 5cm lengths and toss with the dressing.
