Thinly slice the chicken. Add salt, pepper, light soy sauce, oyster sauce, cornstarch, and water. Mix well until each piece is well coated.
After mixing, add round 1 tbsp. cooking oil in and mix well again. Set aside.
Prepare the Bok Choy
Wash the Bok Choy carefully to remove any sandy and then completely drain. Cut into sections.
Stir fry
Heat your wok or pan first. Add the cooking oil, spread the chicken sliced onto the oil when it begins to warm but is not hot. Let them stay for 3-5 seconds and then quickly fry them until become pale. Transfer out immediately.
Keep oil in the wok and fry garlic and Thai chili pepper (if using) until aromatic. Add bok choy, quickly fry for a while. Add light soy sauce, salt and oyster sauce.
Return chicken, add sesame oil and mix well. Transfer out and serve hot.