Place the chicken and oil in a bowl and toss to combine. Heat a large frying pan over a high heat until hot. Add half the chicken and cook for two minutes on one side until lightly browned, turn and cook for another minute. Remove from the pan and repeat with the remaining chicken.
Reduce the heat to medium and add a little extra oil if needed. Add the onion and garlic and cook for five minutes, stirring occasionally. Return the chicken to the pan, season liberally with black pepper, add the soy sauce and stir to combine.
Cover the pan, reduce heat to low and cook for 10 minutes, stirring occasionally. Increase the heat to high, add the sugar and stir to combine. Cook uncovered for 3-4 minutes, or until the sauce is rich, dark and syrupy.
Add the fish sauce and stir to combine. Place in a serving dish and serve with steamed rice and green vegetables.