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 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, garlic, and ginger, and stir-fry for 2-3 minutes until fragrant.
Add the sliced bell pepper to the skillet and continue to stir-fry for another 2 minutes until the vegetables are tender-crisp.
Return the cooked beef to the skillet. Add the oyster sauce, rice vinegar, and sesame oil. Stir everything together and cook for an additional 2 minutes. Season with salt and pepper to taste.
Remove from heat and garnish with chopped green onions. Serve hot over cooked rice or noodles.