In a medium bowl, combine the sliced chicken breast with 1 tbsp light soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, and 1/4 tsp white pepper. Mix well to coat the chicken evenly. Set aside to marinate for at least 15 minutes while you prepare other ingredients.

In a small bowl, whisk together 3 tbsp light soy sauce, 1 tbsp dark soy sauce, 2 tbsp oyster sauce, 1 tbsp granulated sugar, 1/2 tsp sesame oil, 1/4 tsp white pepper, 1/2 cup chicken broth, and 1 tbsp cornstarch until smooth. Set aside.

Bring a large pot of water to a rolling boil. Add the chow mein noodles and cook according to package directions until al dente (usually 2-3 minutes for fresh noodles). Drain immediately and rinse briefly with cold water to prevent sticking. Toss with 1 tsp of vegetable oil to prevent clumping and set aside.

Heat 1 tbsp of vegetable oil in a large wok or skillet over high heat until smoking lightly. Add the marinated chicken in a single layer and stir-fry for 2-3 minutes until lightly browned and cooked through. Remove the chicken from the wok and set aside.

Add the remaining 1 tbsp of vegetable oil to the wok. Add minced garlic and grated ginger and stir-fry for 15-30 seconds until fragrant.

Add the sliced yellow onion, shredded cabbage, julienned carrot, and sliced celery to the wok. Stir-fry for 3-4 minutes until the vegetables are crisp-tender.

Add the white parts of the green onions and the bean sprouts to the wok. Stir-fry for another 1 minute.

Return the cooked chicken to the wok with the vegetables. Add the cooked noodles.

Give the prepared chow mein sauce a quick whisk (as cornstarch may settle) and pour it over the noodles and vegetables. Toss everything together vigorously using tongs, ensuring the noodles and ingredients are evenly coated with the sauce. Continue to stir-fry for 2-3 minutes until the sauce thickens and coats everything beautifully.

Stir in the green parts of the green onions just before serving. Serve hot and enjoy your homemade Chicken Chow Mein!