1 pound (450g) leafy green vegetables, such as curly spinach, watercress, Swiss chard, turnip or beet greens, or bok choy, washed well (see note)
1 tablespoon (15ml) mirin
1/4 cup (60ml) soy sauce, preferably usukuchi (light) soy sauce
3/4 to 1 cup (175 to 235ml) homemade or instant dashi
Bonito flakes or sesame seeds, for garnish
Prepare an ice bath in a large bowl. In a medium or large pot of salted boiling water, cook greens until tender, 1 to 2 minutes depending on the type. Immediately transfer to the prepared ice bath to cool.
Meanwhile, in a small saucepan, bring mirin and soy sauce to a simmer over medium-high heat, then remove from heat. Combine soy mixture with 3/4 cup (175ml) dashi. Taste, then add more of remaining dashi if a more dilute flavour is desired. Let cool completely.
Drain greens. Using your hands, wring out as much water from greens as possible, then transfer to a container, pour dashi marinade on top, and toss to coat. Refrigerate, covered, for at least 30 minutes and up to 8 hours.
Arrange greens on a single serving plate or several individual ones; they are often arranged in a neat line, or compressed into a cylinder shape, though loosely mounding them also works. Drizzle some of the dashi marinades on top, then garnish with bonito flakes or sesame seeds. Serve.