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.
