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 smooth. Set aside.
03 - Bring a pot of water to a boil. Add broccoli florets and cook for 1 minute until bright green and crisp-tender. Drain immediately and set aside.
04 - Heat 1 tbsp 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. Remove beef to a plate.
05 - Add remaining 1 tbsp 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 crisp.
07 - Return beef to the wok. Pour in prepared sauce and toss everything together. Stir-fry for 1-2 minutes until sauce thickens and coats all ingredients evenly.
08 - Transfer to a serving platter. Sprinkle with toasted sesame seeds and fresh cilantro or extra spring onions if desired. Serve immediately while hot.