Use a fork to steady your onion. Slice the onion into rings about 1cm wide. Remove the skin and separate the rings.
Heat the oil to 180C in a heavy-based pan – it should be no more than 1/ 3 full.
Meanwhile put the flour and sparkling water in a bowl and season generously. Whisk together to form a batter.
Coat a small batch of onion rings in batter. Carefully lower into the hot oil and deep-fry until crisp and golden, about 2 - 3 minutes. Remove with a slotted spoon and place on a piece of kitchen towel to drain. Repeat with the remaining onion and batter.