In a mixing bowl, combine sliced flank steak with half the soy sauce, cornstarch, and half the black pepper. Mix thoroughly and marinate for at least 15 minutes.

Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat. Add sliced onions and bell peppers. Stir-fry for 3-4 minutes until slightly tender.

In the same skillet, add the marinated flank steak and sear for 1 minute. Stir-fry until cooked through for an additional 2-3 minutes.

Prepare the sauce by mixing the remaining soy sauce, water, minced garlic, and sugar in a bowl. Pour over cooked steak, add vegetables, and cook together for 2 minutes.