Make the dipping sauce by mixing the soy, sugar and rice wine together. Squeeze the ginger in a garlic press to extract the juice and add to the sauce. Beat the water, egg, flour, cornflour and salt together. The batter will not be smooth. Heat the oil to medium hot, about 160C/325F. 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. 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.