Prepare the Beef: Place the thinly sliced beef in a medium bowl. Add the cornflour and toss thoroughly with your hands or tongs until every piece is evenly and generously coated. Shake off any excess.

Mix the Sauce: In a separate small bowl or jug, whisk together all the sauce ingredients: light soy sauce, dark soy sauce, brown sugar, water, sesame oil, and optional red chilli flakes. Set aside.

Heat the Oil: Pour the vegetable oil into a large wok or heavy-bottomed frying pan and place it over a medium-high heat. The oil is ready when a small piece of beef sizzles immediately.

Fry the Beef: Carefully add half of the coated beef to the hot oil, ensuring not to overcrowd the pan. Fry for 2-3 minutes, turning occasionally, until golden brown and crisp. Use a slotted spoon to remove the beef and place it on a plate lined with a paper towel. Repeat with the remaining half of the beef.

Start the Aromatics: Carefully pour out most of the oil from the pan, leaving about 1 tablespoon behind. Return the pan to a medium heat. Add the minced garlic, grated ginger, and the white parts of the spring onions. Stir-fry for about 30-45 seconds until fragrant, being careful not to let the garlic burn.

Create the Sauce: Give the sauce mixture a final stir and pour it into the pan. Bring it to a simmer, stirring continuously. Let it bubble away for 2-3 minutes. What works best for me is watching for the sauce to visibly thicken into a glossy glaze that can coat the back of a spoon.

Combine and Serve: Return the crispy beef to the pan with the sauce. Add the green parts of the spring onions. Gently toss everything together for about 1 minute, until the beef is thoroughly coated in the glorious, sticky sauce.

Garnish and Enjoy: Transfer the Mongolian Beef to a serving dish, sprinkle with toasted sesame seeds, and serve immediately with steamed rice.