Toast the White Sesame Seeds in a dry skillet over medium heat, shaking the pan frequently, until fragrant and lightly golden (about 2–3 minutes); transfer to a bowl to cool.
Grind most of the toasted seeds to a coarse paste in a mortar and pestle or spice grinder, reserving a little for garnish. Stir in Granulated Sugar, Soy Sauce and Mirin until smooth and well combined; taste and adjust the balance of sweetness and saltiness.
Bring a large pot of water to a boil and add a pinch of Salt. Blanch the Spinach for 20–30 seconds, just until bright green and tender.
Immediately plunge the spinach into ice water to stop the cooking, then drain and gently squeeze out excess water with your hands or a clean cloth. Trim stems if necessary and cut the spinach into 1–2 inch pieces.
Toss the squeezed spinach with the sesame dressing until evenly coated. Sprinkle the reserved toasted sesame seeds over the top for texture and aroma.
Serve the goma-ae at room temperature or slightly chilled as a refreshing side dish. Adjust seasoning with a little extra soy or salt if needed before serving.
