In a medium bowl, combine the chicken pieces with 1 tablespoon of soy sauce and 1 tablespoon of cornstarch. Toss to coat evenly. Let it marinate for at least 15 minutes.

Bring a large pot of water to a boil. Add the lo mein noodles and cook according to package directions, about 3-5 minutes. Drain the noodles and rinse them under cold water. Toss with a little sesame oil to prevent sticking.

In a small bowl, whisk together the remaining soy sauce, oyster sauce, sesame oil, rice vinegar, brown sugar, cornstarch, ground ginger, minced garlic, chicken broth, and red pepper flakes (if using).

Heat 1 tablespoon of vegetable oil in a large wok or skillet over medium-high heat. Add the marinated chicken and stir-fry until cooked through, about 5-7 minutes. Remove the chicken from the wok and set aside.

Add the remaining 1 tablespoon of vegetable oil to the wok. Add the carrots, celery, mushrooms, and red bell pepper and stir-fry for 3-5 minutes, or until the vegetables are tender-crisp. Add the green onions and bean sprouts (if using) and stir-fry for another minute.

Pour the sauce over the vegetables and bring to a simmer, stirring constantly until the sauce thickens, about 1-2 minutes.

Add the cooked chicken and noodles to the wok and toss everything together to coat evenly with the sauce. Cook for another minute or two, until everything is heated through.

Serve immediately. Garnish with extra green onions or sesame seeds, if desired.