In a medium bowl, combine the soy sauce, cornstarch, and vegetable oil. Add the thinly sliced flank steak and toss to coat. Let marinate for at least 15 minutes.
Prepare the Stir-Fry Sauce:
In a small bowl, whisk together the beef broth, soy sauce, oyster sauce, hoisin sauce, rice vinegar, sugar, and black pepper. Set aside.
Cook the Steak:
Heat 2 tablespoons of vegetable oil in a large skillet or wok over high heat. Add the marinated steak in a single layer. Cook for about 2-3 minutes, or until browned and cooked through. Remove the steak from the skillet and set aside.
Stir-Fry the Vegetables:
In the same skillet, add a bit more oil if needed. Add the sliced onions and bell peppers. Stir-fry for about 3-4 minutes, or until the vegetables are tender-crisp.
