Start by cutting the chicken breast into bite-sized pieces and set aside.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and ginger. Stir-fry for about 30 seconds until fragrant.
Add the chicken pieces to the skillet and stir-fry until they are cooked through, about 5-7 minutes.
Once the chicken is cooked, add the sliced bell pepper and onion. Continue to stir-fry for another 3-4 minutes until the vegetables are tender yet crisp.
In a small bowl, mix together soy sauce, black pepper, oyster sauce (if using), and sugar. Pour this mixture over the chicken and vegetables.
Stir to coat everything evenly and cook for an additional 2 minutes, allowing the sauce to thicken slightly.
Remove from heat and garnish with chopped green onions before serving.