01 - Combine the sliced flank steak with 2 tbsp soy sauce, Shaoxing wine, 1 tsp cornstarch, and 1/2 tsp black pepper in a bowl. Mix thoroughly to coat each piece. Let marinate at room temperature for 10-15 minutes while preparing other ingredients.
02 - Whisk together 2 tbsp soy sauce, oyster sauce, hoisin sauce, sugar, beef broth, and 1 tsp cornstarch in a small bowl until cornstarch dissolves completely. Set aside until needed.
03 - Heat 1 tbsp vegetable oil in a large wok or skillet over high heat until smoking. Add marinated beef in a single layer without overcrowding. Sear for 1-2 minutes until browned on edges but still slightly pink in center. Remove beef from pan and set aside on a plate.
04 - Add remaining 1 tbsp oil to the hot wok. Toss in onion wedges, bell pepper strips, minced garlic, and ginger. Stir-fry vigorously for 2-3 minutes until vegetables are crisp-tender and fragrant.
05 - Return seared beef to the wok with vegetables. Pour in prepared sauce and toss everything together. Continue stir-frying for 1-2 minutes until sauce thickens and coats all ingredients evenly, and beef reaches desired doneness.
06 - Remove from heat. Sprinkle with additional freshly ground black pepper to taste. Serve immediately over steamed white or brown rice.