Prepare an icewater bath. Bring 3 quarts of salted water to a boil. Add the spinach and blanch quickly, 15 to 20 seconds. Drain and plunge into the icewater bath. Drain the spinach well and chop coarsely. Place in a medium serving bowl. Whisk together the sesame oil, vinegar, soy sauce, chili paste, ginger and sugar. Pour the mixture over the spinach and mix to combine. Season with salt and pepper to taste and sprinkle with the toasted sesame seeds. Serve at room temperature or cold.