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 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 minced garlic and stir-fry for about 30 seconds until fragrant.
Add the bell pepper, broccoli, and carrot to the skillet. Stir-fry for about 3-4 minutes until the vegetables are tender-crisp.
Return the beef to the skillet. Add the oyster sauce, sugar, black pepper, and water. Stir well to combine and cook for an additional 2-3 minutes until everything is heated through and the sauce thickens slightly.
Serve hot over cooked rice.