Bring large pot of salted water to boil over high heat. Add spinach and cook 5 minutes. Drain in colander; rinse under cold running water. Gently squeeze dry with hands. Divide spinach evenly amoung 8 salad plates.
Light crush sesame seeds using mortar and pestle. Transfer to small bowl. Add soy sauce, mirin, and sugar and blend until smooth. Spoon over spinach. Serve at room temperature or chilled. Garnish each with a flower.