Clean the spinach, wash thoroughly and drain. Boil up salted water in a large pot. Blanch the spinach for 2-3 minutes. Drain, rinse cold and drain well.
For the sauce, sauté the sesame seeds in a pan. Set aside 2 tablespoons of sesame seeds, grind the rest finely in the universal chopper. Mix the ground sesame, miso paste, soy sauce, sugar and mirin. Peel onion and garlic and dice very finely, mix in.
Arrange the spinach on 4 plates and drizzle with the sesame sauce. Sprinkle with sesame seeds that have been set aside.