Although 3 pounds may seem like too much spinach, the leaves wilt quickly when heated. For best results, toast sesame seeds in a dry skillet over medium-high heat until fragrant and starting to turn golden, shaking pan to color evenly.
Bring the water to a boil in a large saucepan. Add spinach one bunch at a time, covering pan after each addition and adding more spinach once the prior batch has wilted. When all spinach is in the pan, let steam, covered, until completely wilted, about 30 seconds.
Drain spinach in a colander, pressing down with a wooden spoon to extract as much liquid as possible; let cool slightly.
Transfer spinach to a serving bowl, and drizzle with sesame oil, vinegar, and half the toasted sesame seeds. Toss well to combine. Season with salt and pepper. Serve warm, at room temperature, or chilled, and garnish with remaining sesame seeds.