Cut beef tenderloin into thin slices. Season with marinade (A). Lastly add cornflour and sesame oil then parboil the marinated beef slices in hot oil for 10-20 seconds. Once the meat changes colour, dish out and set aside.
Heat 2 tbsp oil in a clean wok, add garlic, Szechuan peppercorns and spring onions. Toss briefly until aromatic. Stir in sauce (B) and bring to a quick boil.
Return the prefried beef to the wok. Toss briefly over high heat until the sauce is fairly dry. Dish out and serve immediately.