Blanch the spinach for 1 minute, then drain and plunge it into iced water to preserve the colour. Drain thoroughly and chop coarsely. When you are ready to serve, stir-fry the spinach in the oil or butter, tossing to ensure it is evenly coated. Turn the spinach into a serving dish, scatter over the sesame seeds and drizzle with the sesame oil.
Serve with grilled or baked fish.