In a bowl, combine the sliced flank steak with soy sauce, oyster sauce, cornstarch, and water. Mix well and let it 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 in a single layer and cook for 2-3 minutes until browned. Remove the steak from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced onion, bell peppers, garlic, and ginger. Stir-fry for about 5 minutes until the vegetables are tender-crisp.
Return the cooked steak to the skillet with the vegetables. Drizzle with sesame oil, and season with salt and pepper. Stir everything together and cook for an additional 2-3 minutes until heated through.
Serve the stir-fry over cooked rice or noodles, garnished with chopped green onions and sesame seeds.