Bring a generous amount of unsalted water to a boil. Add the noodles (all at once) for approximately 2 minutes, or until al dente (toothy but firm). Use chopsticks or two wooden spoons to separate noodles as they cook. Do not overcook.
Empty noodles into a large colander, then immediately flush with cold running water until cool. Shake off excess water and drain for 15 to 30 minutes.
Combine sesame oil, soy sauce, vinegar, sugar and chili oil; mix well. Pour over noodles and use your hands to evenly distribute seasoned sauce. Work carefully so noodles don't break. Gently spread sauce over each strand and allow the noodles to fully absorb sauce before eating.
Add all onions except 2 tablespoons. Toss and mix to evenly distribute the onions. Garnish with remaining onions, sesame seeds and cilantro, if desired.
Noodles taste best if they are allowed to sit and the flavors meld for a few hours before serving.
Serve refrigerated and cold, or at room temperature.
219 calories based on ten servings.