Half-fill a large wok or deep fryer with oil and put on to heat. Break the egg into a bowl, add a pinch of salt and whisk until frothy. Quickly whisk in the mineral water, then the flour – you are not aiming for a smooth batter.
Mix the sugar and cinnamon together on a saucer and put aside. Peel, core and slice the apples into rings about 5mm thick.
When the oil has reached a temperature of 190°C, and a few drops of the batter sizzle as soon as they are dropped in, dip each of the apple slices into the batter with a fork and drop into the oil. Deep-fry the fritters in two or three batches for 1–2 minutes each, or until puffed and golden, turning carefully with a slotted spoon halfway through cooking.
Drain on paper towels and serve immediately, sprinkled with the spiced sugar.