In a mixing bowl, combine cubed chicken thighs with soy sauce, oyster sauce, sesame oil, Chinese five spice, and brown sugar. Toss until the chicken is well coated, then cover and let it marinate for 15 minutes at room temperature.

While the chicken marinates, prep your ingredients by slicing the ginger thinly, mincing the garlic, and cutting the onion into wedges.

Heat a tablespoon of oil in a large wok or skillet over medium heat. Add the sliced ginger and sauté for about 30 seconds until fragrant, then add minced garlic and stir for another 30 seconds.

Increase the heat to high and add the marinated chicken in a single layer. Cook undisturbed for 3-4 minutes until nicely browned, then flip and add onion wedges, cooking for another 3-4 minutes.

Reduce heat to medium, pour in the remaining marinade and a splash of water mixed with cornstarch. Stir and let simmer for about 2 minutes until sauce thickens.

Stir in reserved green parts of spring onions and allow to wilt slightly for about 1 minute before serving.