Begin by slicing the sirloin steak thinly against the grain, ensuring tenderness in your Black Pepper Beef. Season the slices with salt and freshly ground black pepper, coating them evenly.

In a large wok or frying pan, heat a combination of sunflower oil and sesame oil over high heat until shimmering, about 1-2 minutes. Add the seasoned steak slices in a single layer, avoiding overcrowding.

Using the same wok, add the sliced onions and bell peppers, stirring them quickly over high heat. Cook for approximately 2 minutes until the vegetables are slightly softened yet still crisp.

In a jug, mix together cornflour, dark soy sauce, oyster sauce, rice wine, and beef stock, whisking until smooth. Pour the sauce mixture directly into the wok, ensuring it spreads evenly among the vegetables.

Add the minced garlic and ginger to the wok, stirring well to combine with the sauce and vegetables. Allow the mixture to simmer gently for 2-3 minutes, or until the sauce thickens and becomes glossy.

Once everything is well combined and heated through, remove the wok from the heat. Serve your delicious Black Pepper Beef over fluffy jasmine rice or garlic noodles, and garnish with scallions and sesame seeds.