Pat the chicken pieces dry with paper towels and set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and sliced onion to the skillet and sauté for 2-3 minutes until fragrant and the onions are translucent.
Add the chicken pieces to the skillet and cook for 5-7 minutes, stirring occasionally, until the chicken is browned on all sides.
Sprinkle the freshly ground black pepper over the chicken and stir well to coat evenly.
Add the soy sauce, oyster sauce, sugar, and salt to the skillet. Stir to combine all ingredients.
Pour in the water and bring the mixture to a simmer.
Add the sliced green and red bell peppers to the skillet and cook for another 5 minutes until the peppers are tender but still crisp and the chicken is cooked through.
Sprinkle the chopped spring onions over the dish and give it a final stir.
Remove from heat and serve hot with steamed rice.