01 - Combine sliced beef with soy sauce, cornstarch, and sesame oil in a bowl. Mix thoroughly and let marinate for 10 minutes at room temperature.
02 - Whisk together soy sauce, oyster sauce, hoisin sauce, rice vinegar, sriracha, brown sugar, and water in a small bowl until sugar dissolves completely.
03 - Bring a pot of water to boil. Blanch broccoli florets for 1 minute until bright green and crisp-tender. Drain immediately and set aside.
04 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Add marinated beef and stir-fry for 2-3 minutes until browned on edges. Transfer beef to a plate.
05 - Add remaining 1 tablespoon oil to the wok. Add garlic, ginger, and spring onions. Stir-fry for 30 seconds until fragrant.
06 - Add bell pepper and blanched broccoli to the wok. Stir-fry for 2 minutes until vegetables are heated through but still retain crunch.
07 - Return beef to the wok. Pour prepared sauce over ingredients. Toss everything together and stir-fry for 1-2 minutes until sauce thickens and coats all components evenly.
08 - Transfer to serving plates. Garnish with toasted sesame seeds and fresh cilantro or extra spring onions. Serve immediately while hot.