In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let it marinate for about 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced onion and bell pepper, and stir-fry for about 2-3 minutes until they begin to soften.
Add the minced garlic and stir-fry for an additional 30 seconds until fragrant.
Return the cooked beef to the skillet, then add the oyster sauce, sesame oil, salt, and pepper. Stir everything together and cook for another 2 minutes until heated through.
Serve the stir-fry over cooked rice.