In a bowl, combine the thinly sliced flank steak, soy sauce, and cornstarch. Mix thoroughly until the steak is well-coated and let it marinate for about 10 minutes.

Heat a large skillet over medium-high heat and add 2 tablespoons of vegetable oil. Once the oil shimmers, add the marinated steak in a single layer. Sear the steak for about 5 minutes, stirring occasionally, until it's browned and slightly crispy.

In the same skillet, add the sliced red and green bell peppers, along with the sliced onion and minced garlic. Cook, stirring frequently, for about 4 minutes until the vegetables are tender-crisp and vibrant in color.

Once the veggies are ready, return the cooked steak to the skillet. Add 1 tablespoon of oyster sauce and 1 teaspoon of black pepper, stirring well to coat everything evenly. Cook for an additional 2 minutes.

Remove the skillet from heat and scoop the sizzling mixture onto plates. Serve your Sizzling Chinese Pepper Steak with Onions hot over a bed of steamed rice or noodles.