01 - Combine sliced beef with 1 tablespoon soy sauce, 1 tablespoon cornstarch, and 1 teaspoon sesame oil in a bowl. Toss thoroughly to coat and let marinate for 10 minutes at room temperature.
02 - Whisk together all sauce ingredients in a small bowl: soy sauce, oyster sauce, hoisin sauce, cornstarch, brown sugar, broth, rice vinegar, and black pepper until smooth and well combined. Set aside.
03 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Add broccoli florets and red bell pepper, stir-frying for 2 to 3 minutes until bright green and crisp-tender. Transfer vegetables to a plate and set aside.
04 - Add remaining 1 tablespoon oil to the pan. Arrange marinated beef in a single layer without overcrowding. Sear for 1 to 2 minutes per side until browned but just cooked through. Work in batches if necessary.
05 - Add minced garlic and ginger to the pan with the beef. Stir-fry constantly for 30 seconds until fragrant, being careful not to burn the garlic.
06 - Return the cooked vegetables to the pan. Stir everything together, then pour in the prepared garlic sauce. Cook for 1 to 2 minutes, stirring constantly, until the sauce thickens and coats all ingredients evenly.
07 - Transfer immediately to serving plates and serve hot alongside steamed white rice or noodles for a complete meal.