In a bowl, combine the thinly sliced flank steak with soy sauce and cornstarch. Mix well and let it marinate for about 15-20 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat. Add the marinated steak in a single layer and sear for 1-2 minutes until browned. Remove the beef from the pan and set aside.
In the same skillet, add the remaining tablespoon of oil. Add the sliced onions and bell peppers. Stir-fry for 3-4 minutes until they begin to soften.
Add minced garlic and ginger to the pan, stirring for another minute until fragrant.
Return the steak to the skillet, and stir to combine with the vegetables.
Add oyster sauce, rice vinegar, and sugar. Stir well to coat the beef and vegetables in the sauce. Cook for another 2-3 minutes or until the sauce is slightly thickened.
Season with salt and black pepper to taste.
Remove from heat, and transfer to a serving dish.
Notes
Serve hot over steamed rice or noodles, garnished with sliced green onions for a pop of color.