- In a bowl, whisk together cornflour, dark soy sauce, oyster sauce, rice wine, beef stock, and 1 tsp black pepper until smooth. Set sauce aside. 
- Season sliced beef with salt and remaining black pepper. 
- Heat 2 tbsp sunflower oil and sesame oil in a large wok or skillet over high heat. 
- Add onions and stir-fry for 2-3 minutes until softened. 
- Add seasoned beef, garlic, and ginger to the pan. Stir-fry for 3-4 minutes until beef is browned. 
- Toss in bell peppers and stir-fry for 2-3 minutes until just tender but still crisp. 
- Pour the prepared sauce into the pan and cook for 1-2 minutes, stirring constantly until sauce thickens and coats the beef. 
- Taste and adjust seasoning with additional salt and black pepper if desired. Serve immediately while hot.