Steps
In a small bowl, mix soy sauce, oyster sauce, sesame oil, cornstarch, and water to create the sauce. Set aside.
Heat vegetable oil in a skillet or wok over medium-high heat. Add chicken; stir-fry until lightly golden and mostly cooked, about 2-3 minutes.
Add garlic and ginger; stir-fry for 30 seconds until aromatic.
Add broccoli, bell pepper, and onion. Stir-fry for 2-3 minutes until vegetables are crisp-tender.
Pour the sauce over the chicken and vegetables; stir well and cook for another minute until the sauce thickens and everything is coated.
Transfer to a plate, sprinkle with toasted sesame seeds if desired, and serve hot.