In a large mixing bowl, combine the thinly sliced flank steak, soy sauce, and cornstarch. Mix well to coat the beef and let it marinate for about 15 minutes.
Heat 1 tablespoon of vegetable oil in a wok or large frying pan over high heat.
Add the marinated beef in a single layer and stir-fry for about 3-4 minutes until browned and cooked through. Remove the beef from the pan and set it aside.
In the same pan, add the remaining tablespoon of vegetable oil. Stir in the sliced onions, red bell pepper, and green bell pepper. Stir-fry for about 3-4 minutes until the vegetables are slightly tender but still crisp.
Add the minced garlic and ginger to the pan and stir-fry for another minute, ensuring they don't burn.
Return the cooked beef to the pan and sprinkle in the coarsely ground black pepper. Stir everything together for an additional 2 minutes to combine the flavors.
Transfer the sizzling pepper steak with onions to serving plates and garnish with chopped green onions. Serve immediately.
For a spicier kick, you can add a few slices of fresh chili peppers or a dash of chili sauce.
Serve with steamed rice or noodles for a complete meal.
Ensure the beef is thinly sliced against the grain for maximum tenderness.