For green chilli dressing, blend ingredients with 1½ tsp salt, ½ tsp finely ground black pepper and 150ml water in a blender until smooth.
Combine prawns, onion and cucumber with chilli dressing in a non-reactive bowl. Cover, then refrigerate to cure prawns (4-6 hours for a light cure; up to 1 day for a medium cure).
Serve chilled ceviche with extra lime juice to taste.
