Prepare the beef: In a mixing bowl, toss the thinly sliced beef with the cornflour until every piece is lightly coated. You should see a fine, powdery white layer that disappears as you work it in. The beef will look dusty but feel slightly tacky.

Mix the sauce: In a small bowl, whisk together the beef stock, soy sauce, brown sugar, rice vinegar, sesame oil, chilli flakes (if using), and white pepper. Stir until the sugar has completely dissolved—the liquid will turn a deep amber colour and smell sweet and savoury.

Heat the skillet: Place a large frying pan or wok over high heat and add 2 tablespoons of oil. Wait until the oil shimmers and a single piece of beef sizzles instantly when dropped in—this should take about 1 minute.

Sear the beef: Add the beef in a single layer (work in batches if needed). Let it cook undisturbed for 1 minute. You'll hear a loud sizzle and see the edges turn golden brown. Flip and cook for another 30–45 seconds. The beef should be browned on the outside but still tender inside. Remove to a plate.

Sauté aromatics: Lower the heat to medium and add the remaining 1 tablespoon of oil. Add the garlic and ginger. Stir constantly for about 20 seconds—the kitchen will fill with a fragrant, sharp aroma, and the garlic should soften without colouring.

Simmer the sauce: Pour the sauce mixture into the pan. Increase the heat to high and bring it to a bubbling boil. Let it cook for 2–3 minutes, stirring occasionally. The sauce will thicken and become syrupy, coating the back of a spoon. You'll see large, glossy bubbles forming.

Combine everything: Return the beef to the pan along with the sliced spring onions. Toss everything together for 30–60 seconds until the sauce clings to each strip of beef. The onions will soften slightly but remain vibrant green. The meat should look lacquered and glossy.

Serve immediately: Transfer to a warm serving plate. The sauce will continue to thicken as it sits, so serve right away over steamed jasmine rice or noodles.