Combine dashi, soy sauce, and mirin in a small bowl.
Cook spinach in a large pot of boiling salted water just until wilted and bright green, a matter of seconds.
Drain and transfer to a large bowl of lightly salted ice water; let cool.
Drain and squeeze out excess liquid from greens.
Pack into a cylinder shape.
Cut in half crosswise or into bite-size lengths.
To serve, divide greens among bowls and top with broth.
Scatter bonito flakes over top.
