Heat the vegetable oil in a large pan or wok over medium-high heat.
Add the minced ginger and garlic. Sauté until fragrant, about 1-2 minutes.
Add the chopped onions and cook until they become translucent and start to brown, about 5 minutes.
Add the chicken pieces to the pan and cook until they are no longer pink, about 6-8 minutes, stirring occasionally.
Add the sliced red and green bell peppers. Cook for an additional 3-4 minutes until the peppers are slightly tender but still crisp.
In a small bowl, mix together the soy sauce, oyster sauce (if using), hoisin sauce (if using), rice vinegar, sugar, and black pepper.
Pour the sauce over the chicken and vegetables. Stir well to coat everything evenly.
Cook for another 2-3 minutes, allowing the sauce to thicken slightly and the flavors to meld together. Add chili flakes if using.
Garnish with sliced green onions.
Enjoy your meal!