Combine cornflour, salt and pepper in a large bowl. Add half of the prawns. Toss to coat, shaking of excess.
Heat half the oil in a large, non-stick frying pan over high heat. Cook prawns, stirring, for 3 to 4 minutes or until pink. Repeat with remaining cornflour mixture, prawns and oil.
Place prawns on a plate. Top with coriander, chilli and onion. Serve.