In a bowl, mix the flank steak with 2 tablespoons soy sauce and cornstarch. Let it marinate for at least 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat. Add the beef in a single layer and sear until browned, about 1-2 minutes per side. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Stir in the green and red bell peppers, onion, garlic, and ginger, and cook for about 3-4 minutes, or until just tender.
Return the beef to the skillet. Add beef broth, oyster sauce, 1 tablespoon soy sauce, and sugar. Cook, stirring frequently, until the sauce has thickened and the beef is cooked through, about 2-3 minutes.
Season with salt and pepper to taste. Serve immediately.