Begin by slicing the flank steak against the grain into thin strips. In a mixing bowl, combine steak, soy sauce, and cornstarch. Allow to marinate for 10 minutes.

Heat a large skillet over medium-high heat and add vegetable oil. Once shimmering, add the marinated steak in a single layer and stir-fry for about 5 minutes until browned and cooked through. Transfer to a plate and set aside.

In the same skillet, add another tablespoon of vegetable oil. Add chopped red and green bell peppers and sliced onion, stirring constantly for 3-4 minutes until tender-crisp. Add minced garlic and stir until fragrant.

Return the cooked beef to the skillet with the vegetables. Pour in oyster sauce and sprinkle with black pepper. Stir to combine for another 2 minutes until everything is well coated.

Remove from heat and serve immediately over jasmine rice or noodles for a complete meal.