Heat the vegetable oil in a large pan or wok over medium-high heat.
Add the chicken strips and cook until browned and fully cooked, about 5 minutes. Remove and set aside.
In the same pan, add garlic and ginger, sauté for 30 seconds.
Add the bell peppers and broccoli, stir-frying for 3-4 minutes until tender-crisp.
Return the chicken to the pan, add soy sauce, oyster sauce, and red pepper flakes (if using). Stir well.
Pour in the cornstarch mixture and stir until the sauce thickens, about 1 minute.
Drizzle with sesame oil, give it a final stir, and serve hot over cooked rice.