Put the slices into a large bowl of water and set aside for 30 minutes. Drain and dry thoroughly on kitchen towels.

Fill two large saucepans one-third full with oil. Heat the oil in one pan until it reaches 170°C (325 °F) on a deep-fat thermometer, or until a small cube of stale bread dropped into the oil turns golden in 65 seconds. Heat the oil in the second pan until it reaches 190°C (375 °F) on a deep-fat thermometer or until a small cube of stale bread dropped into the oil turns golden in 40 seconds.

Drop the potato slices into the first pan and fry for 4 minutes. Using a slotted spoon, transfer the slices to the second pan and fry for 2 to 3 minutes or until they puff up. Remove from the oil and drain on kitchen towels.

Sprinkle over the salt and serve at once.