In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef in a single layer and cook for 2-3 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 cook for 2 minutes until softened.
Add the minced garlic, bell pepper, and broccoli florets to the skillet. Stir-fry for another 3-4 minutes until the vegetables are tender-crisp.
Return the beef to the skillet and pour in the oyster sauce and sesame oil. Stir everything together and cook for an additional 2 minutes. Season with salt and pepper to taste.
Serve the stir fry over cooked rice or noodles.