In a bowl, mix the chicken with cornstarch and oil until lightly coated.
In a separate bowl, combine all sauce ingredients and set aside.
Soak the rice noodles in hot water according to package instructions, then drain well.
Heat oil in a wok or large skillet over high heat.
Add garlic and onion, stir-frying for about 30 seconds until fragrant.
Add the chicken and cook for 1 minute until it turns white.
Add carrot, choy sum stems, and bell pepper. Stir-fry for 1 minute.
Add mushrooms and cook for another 30 seconds.
Add noodles, choy sum leaves, and sauce. Toss gently for about 1 minute until everything is well coated and the sauce reduces.
Garnish with sesame seeds and green onions. Serve immediately.