01 - Toss the sliced beef with cornstarch and 1 tablespoon soy sauce in a bowl. Set aside to marinate while preparing other ingredients.
02 - Whisk together all sauce ingredients in a separate bowl: soy sauce, oyster sauce, chili garlic sauce, rice vinegar, brown sugar, and sesame oil until smooth and combined.
03 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Add beef in a single layer and sear for 1-2 minutes per side until browned. Transfer to a plate.
04 - Add remaining 1 tablespoon oil to the wok. Sauté garlic, ginger, and white parts of spring onions for 30 seconds until fragrant.
05 - Add broccoli and red bell pepper to the wok. Stir-fry for 2-3 minutes until vegetables are bright and beginning to soften.
06 - Return beef and accumulated juices to the pan. Pour in sauce and stir thoroughly. Cook 2-3 minutes until sauce thickens and everything is heated through.
07 - Remove from heat, sprinkle with green parts of spring onions, and serve immediately with steamed rice or noodles.