01 - Toss the sliced beef with cornstarch and 1 tablespoon soy sauce in a bowl. Let marinate at room temperature while preparing other ingredients.
02 - Whisk together soy sauce, oyster sauce, chili garlic sauce, rice vinegar, brown sugar, and sesame oil in a separate bowl until sugar dissolves completely.
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 to 2 minutes per side until just browned. Transfer to a plate.
04 - Add remaining oil to the wok. Sauté garlic, ginger, and white spring onion parts for 30 seconds until fragrant, stirring constantly.
05 - Add broccoli and red bell pepper. Stir-fry for 2 to 3 minutes until vegetables are bright green and beginning to soften.
06 - Return beef and accumulated juices to the pan. Pour in sauce and stir to coat everything evenly. Cook 2 to 3 minutes until sauce thickens and ingredients are heated through.
07 - Remove from heat. Garnish with green spring onion parts. Serve immediately over steamed jasmine rice or rice noodles.