Marinate the chicken with 1 tbsp soy sauce, cornstarch, and 1 tbsp oil. Set aside for 15 minutes.
Heat 1 tbsp oil in a large skillet over medium heat. Add the marinated chicken and cook until golden brown. Remove from the skillet and set aside.
In the same skillet, add garlic and sauté until fragrant. Add the onion and bell pepper, cooking until slightly tender.
Return the chicken to the skillet. Stir in the remaining soy sauce, oyster sauce, sugar, and black pepper. Mix well.
Cook for another 3–5 minutes, ensuring the chicken is fully coated in the sauce. Serve hot with steamed rice or noodles.