Combine the lamb, garlic, ginger, five-spice powder, chilli flakes and 2 tablespoons of the soy sauce in a bowl.
Spray a large nonstick wok or frying pan with cooking spray and heat over a high heat until just smoking. Add the onion and green beans and stir-fry for about 3–4 minutes until lightly charred and tender. Transfer to a bowl.
Wipe the pan clean with kitchen paper, re-spray with cooking spray and heat over a high heat. Add half the lamb mixture and stir-fry for 2 minutes, or until browned and just cooked through. Transfer to a plate. Wipe the pan clean, spray again with cooking spray and stir-fry the rest of the lamb mixture in the same way.
Return all the lamb mixture to the pan with the onion mixture and add the remaining soy sauce and the kecap manis. Cook, stirring, for about 30 seconds until heated through.
Divide between 4 warmed bowls, garnish with chilli slivers and cucumber ribbons and serve with egg noodles.