Heat a wok or large skillet over high heat until hot.
Add the vegetable oil and reduce the heat to medium.
Add the bean sprouts, sliced chicken, and onion. Stir-fry for about 1 minute.
Add the blanched egg noodles and continue stir-frying for another 30 seconds.
Pour in the dark soy sauce, light soy sauce, and oyster sauce. Season with white pepper.
Toss everything together until the noodles are evenly coated.
Continue stir-frying for about 2 minutes until the noodles become lightly crisp around the edges.
Remove the wok from the heat and drizzle with sesame oil.
Sprinkle with chopped spring onion.
Serve the Simple Chicken Chow Mein immediately while hot.