Combine the cornflour with the rice wine to make a paste, then stir in the soy sauce and garlic. Add the lamb and stir to cover, then leave to marinate for 25–30 minutes. Drain.
Place the Szechuan peppercorns in a dry wok and stir over a medium heat until they begin to pop and release their aroma. Transfer to a pestle and mortar and pound with the salt until coarsely ground.
Heat half the oil in a wok over a high heat until the oil starts to shimmer. Add half the lamb and stir-fry for 3 minutes, then remove using a slotted spoon. Heat the remaining oil and stir-fry the rest of the lamb in the same way.
Return all the lamb to the wok, add the sugar snaps and after 1 minute stir in the sesame oil, chilli, spring onion and the ground salt and pepper mixture. Cook for 1 more minute, then serve with noodles, if liked.