Chill the soda water. Skin and remove the second layer of the large white onions.
- 1 Heat the oil gently in a deep fryer over a medium heat – it will start to shimmer when ready.
- 2 Whisk together the soda water, plain flour, cornflour and a large pinch of salt to form a batter.
- 3 Add the ice cubes to the mixture and keep cool. Put a kettle of water on to boil.
- 4 Slice the onion into rounds about 1cm thick and place in a large heatproof bowl.
- 5 Pour over the boiling water and stir well. Drain and pat dry.
- 6 Dip the onion rings in the batter.
- 7 Drop them into the oil one at a time (cook about 10 in one batch) and increase the heat if necessary so they continue to fry.
- 8 When the onion rings float to the surface, cook for a further 1 minute, then drain on kitchen paper before serving.