In a medium bowl, combine the thinly sliced flank steak with soy sauce, cornstarch, sesame oil, and black pepper. Toss gently and let it sit for 15 minutes.

In a small bowl, whisk together the beef broth, soy sauce, rice vinegar, oyster sauce, brown sugar, ginger, and garlic until smooth.

Heat vegetable oil in a large skillet or wok over medium-high heat. Add half of the marinated beef in a single layer and sear for 2-3 minutes without stirring.

Remove the cooked beef from the skillet and set aside. Add the remaining oil to the skillet and repeat with the leftover beef.

Add the sliced bell peppers and onion to the skillet, stir-fry for 3-5 minutes until crisp-tender.

Return the cooked beef to the skillet, whisk the sauce again, and pour it over the beef and vegetables. Simmer for 1-2 minutes until the sauce thickens.