✓ Slice the flank steak thinly against the grain into bite-sized strips. Place in a bowl.

✓ Add 2 tablespoons soy sauce and 1 tablespoon cornstarch to the bowl with the beef. Mix well until the beef is evenly coated. Let marinate for at least 15 minutes.

✓ Prepare the vegetables: Slice the onion, red bell pepper, and green bell pepper into thin strips. Mince the garlic and grate the ginger if using.

✓ In a small bowl, mix together the sauce ingredients: 1/4 cup soy sauce, 2 tablespoons oyster sauce (if using), 1 tablespoon rice vinegar, 1 tablespoon brown sugar, 1 teaspoon sesame oil (if using), and 1/4 teaspoon black pepper. Stir until the sugar is dissolved.

✓ Heat 1 tablespoon vegetable oil in a large skillet or wok over high heat. When the oil is hot, add the marinated beef. Stir-fry until the beef is cooked through, about 3-4 minutes. Remove the beef from the skillet and set aside.

✓ In the same skillet, add a bit more oil if necessary. Add the sliced onion, red bell pepper, and green bell pepper. Stir-fry until the vegetables are tender-crisp, about 3-4 minutes.

✓ Add the minced garlic and grated ginger (if using) to the skillet. Stir-fry for another minute until fragrant.

✓ Pour the prepared sauce into the skillet with the vegetables. Bring to a simmer and cook until the sauce thickens, about 2 minutes.

✓ Return the cooked beef to the skillet. Stir everything together to coat the beef and vegetables with the sauce.

✓ Cook for an additional 1-2 minutes until everything is heated through and well combined.

✓ Remove the skillet from the heat. Give the pepper steak a final stir.

✓ Transfer the pepper steak to a serving platter or individual plates. Serve immediately while hot.