In a deep-sided saucepan, add the oil and bring up to 180°C.
While the oil is coming to temperature, peel and slice the onions whole in 1 cm widths. Separate the layers, keeping the largest ones for rings. Any smaller inner pieces can be put to the side and used for another dish so not to waste them.
In a mixing bowl, add the flour, salt and spices. Mix together.
Add ¼ of the sparkling water to the flour mix and whisk until it forms a paste. Gradually add the remaining sparkling water and mix until the batter resembles a pancake batter. (See photos above).
Check the oil is at the correct temperature with a thermometer. If no thermometer then toss a small piece of bread into the oil, if it starts to sizzle the oil is ready.
Add a few slices of onions at a time to the batter. Coat evenly and using a large fork or tongs remove from the batter, shake off the excess and add to the hot oil. Fry for 3-4 minutes before turning and cooking the other side for 2-3 minutes until golden. Take care not to overcrowd the pan as they will stick together and also reduce the cooking temperature of the oil. Continue to fry the onions in small batches.
As the onion rings become golden, remove them from the oil with the tongs or a slotted spoon. Place in a bowl lined with kitchen paper and allow to drain any excess oil. Repeat for the remaining onions. Serve immediately.