Thinly slice the chicken breast. In a bowl, combine the sliced chicken with cornstarch, salt, and water until the liquid is fully absorbed. Add oil and mix to coat the chicken evenly. Set aside to marinate while you prepare the other ingredients.
Mix the Sauce
In a small bowl, whisk together oyster sauce, light soy sauce, dark soy sauce, cornstarch, sugar, ground white pepper, and water until smooth. Then, stir in sesame oil and set the sauce aside.
Prepare the Noodles
Briefly submerge the precooked noodles in a pot of hot water to warm them up and separate any tangled strands (see note 4 if using uncooked noodles).
Sear the chicken
Heat a wok or large skillet over high heat, then add 1½ tablespoons of oil (if using non-stick cookware, add the oil first, then heat). Put in the marinated chicken, spreading the pieces into a single layer.
Once the bottom side turns pale, flip and toss the chicken until no pink remains. Transfer the chicken to a plate, leaving any oil in the wok.
Stir-fry the dish
In the same wok/skillet, add the remaining 1 tablespoon of oil, followed by garlic and onion, letting them sizzle until fragrant.
Add the bell pepper, mushrooms, and snap peas. Stir-fry over high heat until the vegetables wilt slightly but remain crisp.
Return the seared chicken to the skillet and toss with the vegetables for about 30 seconds.
Then, turn the heat down to medium. Add the noodles and pour in the sauce mixture (be sure to stir the sauce well beforehand). Toss everything to coat evenly and allow the sauce to thicken.
Sprinkle the chopped scallions over the dish and give it one final toss.
Notes
1. You may use other proteins to replace chicken, such as beef, pork, char siu, shrimp, eggs, or tofu. In the “Variations” section inside the main post, you’ll find tips on adjusting the recipe for these alternatives.2. Oyster sauce can be replaced by hoisin sauce. In this case, omit the sugar as hoisin tastes quite sweet.3. Dark soy sauce is used to enhances the color. It’s fine to skip it and use additional light soy sauce instead.4. You may also use uncooked egg noodles (either fresh or dried): bring a pot of water to a boil, then add the noodles. Cook according to package instructions. Drain and briefly rinse under tap water.