Slice the beef thinly against the grain. In a bowl, toss the beef with 1 tablespoon of soy sauce and 1 tablespoon of cornstarch. Let marinate for 15 minutes.

In a small bowl, whisk together the remaining 2 tablespoons of soy sauce, oyster sauce, beef broth, and black pepper to create the sauce.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat. Add the marinated beef in a single layer and sear for 2 to 3 minutes until browned. Remove the beef from the skillet and set aside.

Add the remaining 1 tablespoon of vegetable oil to the skillet. Stir in the minced garlic and ginger, cooking for 30 seconds until fragrant.

Add the sliced bell peppers and onion to the skillet. Stir-fry for 3 to 4 minutes until the vegetables are crisp-tender.

Return the cooked beef to the skillet. Pour the sauce over the mixture and cook for 1 to 2 minutes, stirring constantly, until the sauce thickens and coats all the ingredients. Serve hot.