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.