Put a large pot of water on to boil.
Meanwhile, whisk tahini, vinegar, soy sauce, water and mirin in a large bowl.
Cook spinach in the boiling water until it just turns bright green, 15 to 30 seconds for baby spinach, 45 seconds to 1 minute for mature spinach. Drain in a colander and rinse with cold water. Press or squeeze to remove excess water. Add the spinach to the dressing and toss to coat. Drizzle with sesame oil and sprinkle with sesame seeds, if desired.
