In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Toss to coat and let marinate for 15 minutes.
In a large skillet or wok, heat 1 tablespoon of vegetable oil 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 oil. Sauté the onion and bell pepper for about 3-4 minutes until softened. Add the garlic and ginger, cooking for an additional minute until fragrant.
Return the beef to the skillet and stir in the black pepper, oyster sauce, rice vinegar, sugar, and beef broth. Cook for another 2-3 minutes, stirring frequently, until the sauce thickens and the beef is heated through.
Serve hot, garnished with chopped green onions.