In a large mixing bowl, combine the sliced flank steak, 2 tbsp soy sauce, 1 tbsp cornstarch, sesame oil, and black pepper. Toss well to coat the beef evenly. Let it marinate at room temperature for at least 15 minutes, or refrigerate for up to 30 minutes.

In a small bowl, whisk together the beef broth, 1/4 cup soy sauce, oyster sauce, sugar, rice vinegar, and 1 tbsp cornstarch until smooth. Set aside.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat until shimmering. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding). Cook for 2-3 minutes per side until browned, then remove the beef from the skillet and set aside.

Add the remaining 1 tablespoon of vegetable oil to the same skillet. Add the sliced green bell pepper, red bell pepper, and yellow onion. Stir-fry for 3-5 minutes until the vegetables are crisp-tender.

Add the minced garlic and grated ginger to the skillet with the vegetables. Stir-fry for another minute until fragrant. Return the cooked beef to the skillet with the vegetables.

Give the prepared sauce a quick whisk again (cornstarch settles) and pour it over the beef and vegetables in the skillet. Bring to a simmer, stirring constantly, until the sauce thickens and coats all the ingredients, about 1-2 minutes.

Remove from heat and serve immediately over steamed white rice.