Prepare an ice-water bath; set aside. Bring a large pot of water to a boil; season with salt. Add spinach, and cook just until wilted, about 30 seconds. Drain immediately in a colander, and plunge into prepared ice bath. Let spinach cool completely, about 30 seconds; drain again. Using your hands, squeeze excess water out of spinach, and transfer to a work surface. Coarsely chop spinach, and place in a medium bowl; set aside.
In a small bowl, whisk together sesame oil, soy sauce, rice vinegar, and mirin. Add dressing and sesame seeds to the spinach; mix to combine.
Spinach may be refrigerated for up to 2 days.