Heat the oil in a deep heavy-bottomed saucepan, until very hot or until a breadcrumb sizzles in it. (CAUTION: Hot oil is very dangerous. Do not leave unattended).
Place the flours into a bowl and mix together. Add enough sparkling water to make a thin batter.
Dip the prawns and spring onions into the batter. Carefully place the battered spring onion and prawns into the hot oil and deep-fry until golden and crisp. Drain on kitchen paper.
To serve, transfer on to a serving plate and sprinkle with salt. Serve straight away.
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