Fill a stockpot with water and boil to a boiling add the noodles cook until al dente. 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. Noodles taste best if they are allowed to sit and the flavors meld for a few hours before serving. Top with green onion.