In a small bowl, whisk the cornstarch and dark soy sauce until smooth, then incorporate the remaining sauce components.
Transfer 2 teaspoons of the sauce mixture to a separate bowl and coat the sliced chicken thoroughly.
Heat oil in a wok or heavy skillet over high flame until visibly smoking.
Introduce the onion and garlic; stir-fry briefly for about 30 seconds.
Add the marinated chicken, stirring constantly until the surface turns opaque, leaving the interior slightly raw—approximately 1 minute.
Toss in the carrots and bell pepper, cooking for 2 minutes or until the chicken is cooked through.
Add the prepared noodles, reserved sauce, and water, then mix vigorously using two wooden spoons for 30 seconds.
Stir in the green onions and continue tossing for another minute until the noodles are evenly coated in sauce.
Serve immediately, optionally topped with additional sliced green onions.