In a medium bowl, combine all ingredients for the lo mein sauce. Whisk until the brown sugar is completely dissolved and the mixture is smooth.
Take 1 tablespoon of the prepared sauce and marinate the chicken pieces briefly while preparing the vegetables and noodles.
Bring a pot of water to a boil and cook the egg noodles until just shy of al dente—about 1 to 2 minutes less than package instructions. Drain and rinse under cold water, then lightly coat with oil to prevent sticking.
Note: If using pre-cooked noodles, separate the strands by rinsing them under hot water.
In a wok or large skillet, heat vegetable oil over medium-high heat. Add the marinated chicken and stir fry until fully cooked through. Remove and set aside.
In the same pan, add the sliced vegetables and stir fry for 2 to 3 minutes until tender-crisp.
Return the cooked noodles to the pan along with the prepared sauce. Toss everything together for 1 to 2 minutes, allowing the sauce to thicken and coat the noodles with a glossy finish. Reintroduce the chicken and mix until evenly combined.
Top with chopped scallions and toasted sesame seeds before serving.