Beat the water, egg, flour, cornflour and salt together. The batter will not be smooth. Heat the oil to medium hot, about 160C/325F.
4
Meanwhile, butterfly the peeled prawns by cutting them open through the back, removing the black vein. Cut the spring onions in half across, then in half lengthways.
5
Dip the prawns and onions in the batter and fry a few at a time, draining on kitchen paper. Serve at once with the dipping sauce.