Combine the chicken, garlic, five-spice powder, chilli flakes and 2 tablespoons of the light 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 mangetout 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 the chicken mixture and stir-fry for 4–5 minutes, or until browned and just cooked through. Return the onion and mangetout to the pan and add the remaining light soy sauce and the dark soy sauce. Cook, stirring, for about 2–3 minutes until heated through.
Ladel into warmed bowls, garnish with the coriander leaves, chilli slivers and cucumber ribbons and serve with rice or noodles.