Prepare the Beef: Pat the sliced flank steak dry with paper towels. This ensures a proper sear and prevents steaming. Toss the beef with 1 tablespoon cornstarch and a light pinch of salt and pepper. This coating helps seal in moisture and gives a smooth, velvety texture once cooked.

Mix the Sauce: In a medium bowl, whisk together the soy sauce, oyster sauce, hoisin sauce, rice vinegar, brown sugar, sesame oil, and beef broth. Stir in the cornstarch slurry. Set aside. This combination creates a deeply flavorful, glossy sauce that will coat each ingredient evenly.

Sear the Beef: Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Once the oil is shimmering, add the beef in a single layer. Cook for 1-2 minutes per side, just until browned. Remove and set aside. Avoid overcrowding the pan; cook in batches if necessary.

Stir-Fry the Vegetables: Add the remaining 1 tablespoon of oil to the pan. Stir in the garlic and ginger, sautéing for 30 seconds until fragrant. Add the bell peppers and onions, stir-frying for 2-3 minutes until they begin to soften but still have a slight crunch.

Combine and Simmer: Return the beef to the pan. Pour the sauce over the mixture. Stir well to coat everything evenly. Let the sauce come to a gentle boil, and simmer for 2-3 minutes, until thickened. The cornstarch will help create that signature glossy finish.

Serve: Serve hot over steamed jasmine rice or lo mein noodles. Garnish with sliced green onions or sesame seeds for added flavor and presentation.