In a bowl, combine the chicken with soy sauce, oyster sauce, dark soy sauce, black pepper, and cornstarch. Let marinate for 10 minutes.
Heat a large skillet or wok over medium-high heat. Add 1 tablespoon of vegetable oil and cook the chicken until browned and fully cooked. Remove and set aside.
In the same pan, add the remaining vegetable oil. Sauté the onion, bell pepper, and garlic until tender.
Return the chicken to the pan. Stir everything together and drizzle with sesame oil. Cook for another 2 minutes.
Garnish with green onions and sesame seeds, if desired, and serve hot with steamed rice.