In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for at least 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated steak and stir-fry for about 3-4 minutes until browned. Remove the steak from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Add the sliced bell pepper, onion, garlic, and ginger. Stir-fry for about 3-5 minutes until the vegetables are tender-crisp.
Return the cooked steak to the skillet. Add the beef broth, oyster sauce, rice vinegar, and sugar. Stir everything together and cook for an additional 2-3 minutes until the sauce thickens slightly. Season with salt and pepper to taste.
Serve hot over cooked rice.
Notes
For a spicier kick, add sliced jalapeños or red pepper flakes during the stir-frying step.
Substitute the flank steak with chicken breast or tofu for a different protein option.