Cut the chicken breast into thin slices, about 0.2 inch thick. Mix cornstarch, salt and water in a bowl and add the chicken. Mix very well until the water is fully adsorbed. Stir in oil then leave to marinate for 10 minutes or so.
Prepare the noodles
fresh wide rice noodles often come as a block and the strands are tightly stuck to each other. You'll need to separate them before stir-frying. If you find it hard to do, Review comments in notes section.
Stir-fry the dish
Heat up a wok over high heat until it smokes ( see note 3 if using a wok with non-stick. Pour in 1 tablespoon of oil.
Stir in the chicken. Fry until it loses the pinkness (Do not overcook). Transfer it out, Set aside.
Add the remaining 1 tablespoon of oil into the wok (keep any oil left from frying chicken) Put in the rice noodles. Toss them by moving the wok or gently stir them around using a pair of chopsticks.
One the start to wild (don't wait until they become very soft), add onion, bell pepper and mushrooms. Fry for half a minute or so.
Add the chicken. Pour in light soy sauce, dark soy sauce and sesame oil. Flip and toss to evenly distribute the seasoning (be gentle not to break the noodles). Garnish with scallions and serve immediately.
Notes
You can alternately use dried wide rice noodles. Follow the instruction on the package to cook them. Rinse under cold water then drain well before you start stir-frying.You may use other vegetables, those that cooked easily in a quick stir-fry, items such as Chinese chives, bean sprouts, bok choy, choy sum, snow peas, baby corn, etc.