Heat the wok/pan to medium-high. Add 2 tbsp oil and wait for 30 seconds.
Add marinated chicken and cook for 3-4 minutes (about 90% cooked) and transfer to a plate.
Turn up the heat to high and add 1 tbsp oil to the wok/pan. Add sliced onions, minced garlic and cook for 1 minute or until fragrant.
Add sliced carrots, sliced mushrooms and choy sum stems. Cook for 2 minutes.
Pour the sauce in and mix.
Add chop sum leaves, mung bean sprouts and chicken. Cook for 2-3 minutes or until the sauce has thickened. Remove wok/pan from the heat.