Cut the chicken breast into thin slices, about ¼ inch thick.
In a bowl, mix the cornstarch, salt, and water. Add the sliced chicken and mix well until the liquid is fully absorbed.
Stir in 1 teaspoon of neutral cooking oil. Set aside to marinate for about 10 minutes.
Prepare the Noodles
Fresh wide rice noodles often come in a block with the strands tightly stuck together. Carefully separate the noodles before stir-frying. If they are difficult to separate, see the tips in the Notes section.
If using dried wide rice noodles, prepare them according to the package directions. Rinse under cold water and drain well before stir-frying.
Stir-Fry the Dish
Heat a wok over high heat until very hot. If using a nonstick wok, see the Notes section for the recommended cooking method.
Add 1 tablespoon of oil. Stir in the marinated chicken and stir-fry until the chicken is just cooked through and no longer pink. Do not overcook. Transfer the chicken to a plate and set aside.
Add the remaining 1 tablespoon of oil to the wok, keeping any oil left from cooking the chicken.
Add the rice noodles. Toss them by moving the wok or gently stir them with chopsticks. Avoid stirring too aggressively, as the noodles can break easily.
As soon as the noodles begin to soften, add the onion, bell pepper, and mushrooms. Stir-fry for about 30 seconds.
Return the chicken to the wok. Add the light soy sauce, dark soy sauce, and sesame oil. Gently toss everything together until the noodles and ingredients are evenly coated with the seasoning.
Garnish with chopped scallions and serve immediately.
Notes
Using dried rice noodles: Dried wide rice noodles can be used in place of fresh noodles. Prepare them according to the package directions, then rinse under cold water and drain thoroughly before stir-frying.Separating fresh rice noodles: Fresh wide rice noodles can sometimes be difficult to separate when they come tightly packed. Allowing them to come closer to room temperature can make the strands easier to separate. Gently loosen them with your fingers before adding them to the wok.Vegetable variations: Feel free to substitute or add other vegetables that cook quickly over high heat. Good choices include snow peas, and baby corn.Cooking with a nonstick wok: If using a nonstick wok, avoid heating the empty wok until it is smoking hot. Heat the wok over medium-high heat and add the oil before it becomes excessively hot.