In a bowl, combine the sliced flank steak with cornstarch, soy sauce, and a pinch of salt and pepper. Let it marinate for 15-20 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 2-3 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Add the minced garlic and ginger, and stir-fry for about 30 seconds until fragrant.
Add the sliced bell pepper and broccoli florets to the skillet. Stir-fry for 3-4 minutes until the vegetables are tender-crisp.
Return the beef to the skillet. In a small bowl, mix the oyster sauce, water, and sugar. Pour the sauce over the beef and vegetables, stirring to combine. Cook for an additional 2-3 minutes until everything is heated through and the sauce thickens slightly.
Serve the beef with oyster sauce over cooked rice.