In a mixing bowl, combine the sliced flank steak with soy sauce, oyster sauce, black pepper, and cornstarch. Marinate for about 10 minutes while you prepare the vegetables.
Heat the vegetable oil in a wok or large skillet over medium-high heat.
Once hot, add the marinated beef in a single layer. Stir-fry for about 2-3 minutes, until browned. Remove the beef from the skillet and set aside.
In the same wok, add the sliced onion, red bell pepper, and green bell pepper. Stir-fry for 3-4 minutes until they start to soften.
Add the minced garlic and ginger, cooking for an additional 1 minute until fragrant.
Return the cooked beef to the wok and mix everything together.
In a small bowl, mix the cornstarch with water and add it to the stir-fry. Toss to coat and cook for another 1-2 minutes until the sauce thickens.
Drizzle with sesame oil, if desired, and give everything one last stir.
Serve hot over cooked rice.