Begin by marinating the chicken. In a bowl, combine the soy sauce, oyster sauce, sesame oil, cornstarch, black pepper, and salt. Add the chicken pieces and toss to coat evenly. Let this sit for at least 10 minutes while you prepare the vegetables.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken in a single layer, and allow the chicken to sear undisturbed for 2-3 minutes.

After 3 minutes, stir the chicken, ensuring even cooking. Cook for another 3-4 minutes until browned and cooked through, reaching an internal temperature of 165°F (75°C).

Once cooked, remove from the pan and set aside. In the same skillet, add minced garlic, sliced onion, and bell pepper. Sauté for about 2-3 minutes until tender and fragrant.

If using broccoli florets, add them now and stir-fry for an additional 2 minutes. Then return the cooked chicken to the skillet and toss everything together. Cook for another minute to heat through.

Taste the dish and adjust seasoning if necessary. Remove from heat and garnish with chopped green onions and sesame seeds before serving. Enjoy hot!