Marinate the beef for 30 minutes in a bowl with the sesame oil, soy sauce, salt, sugar, garlic and ginger.
Peel the cucumber, cut it in half lengthways and then into 1cm (½ inch) slices.
Heat half the oil in a wok over a high heat until the oil starts to shimmer. Add half the beef and stir-fry for 2–3 minutes, until just cooked, then remove using a slotted spoon. Heat the remaining oil and stir-fry the rest of the beef in the same way.
Add the cucumber and spring onions. Stir-fry for 1 more minute, until the cucumber is only slightly tender, and serve garnished with a scattering of toasted sesame seeds.