In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let it marinate for 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 3-4 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 sliced onion and bell pepper, and stir-fry for 3-4 minutes until they begin to soften.
Add the minced garlic and cook for an additional 30 seconds until fragrant.
Return the beef to the skillet, and add the oyster sauce, rice vinegar, and sesame oil. Stir well to combine and cook for another 2-3 minutes until everything is heated through.
Season with salt and pepper to taste. Garnish with chopped green onions before serving.