In a mixing bowl, combine the thinly sliced flank steak with the soy sauce and cornstarch. Make sure the steak is evenly coated. Allow it to marinate for 15-20 minutes to absorb the flavors.
Heat sesame oil in a large skillet or wok over medium-high heat until it shimmers, indicating it's hot enough for cooking.
Add the marinated flank steak to the hot skillet, spreading it out into a single layer. Sear for about 2-3 minutes without moving it, allowing it to brown nicely. Once browned, transfer the steak to a plate and set it aside.
In the same skillet, add the vegetable oil. Once heated, sauté the sliced onions and bell pepper for approximately 2-3 minutes. Then, add the minced garlic and ginger, cooking for an additional 2 minutes until the vegetables are tender and fragrant.
Return the seared steak to the skillet, mixing it thoroughly with the sautéed vegetables. Pour in the oyster sauce and sprinkle with black pepper. Stir-fry everything together for another 2 minutes, ensuring the steak is cooked through and the sauce evenly coats all the ingredients.
Remove the skillet from heat. Garnish the dish with freshly chopped green onions before serving.
Notes
Serve with steamed rice or noodles to soak up the delicious sauce.