Sift the flour into a large bowl. Make a well in the flour and pour in the olive oil and stir to combine with the flour. Add the sparkling water and whisk. The batter should be like double cream. if your batter is not this consistency, add a little more water. Place the bowl in the fridge and let the batter rest for around an hour or so. When you are ready to cook, whisk the egg whites until stiff and then fold into the batter mixture. 

While the batter is resting, place the eggplant and zucchini slices in a bowl, cover with a little salt and let them sit for an hour in a colander, with bowl underneath it to collect the water that comes off the vegetables. When you are ready to cook. Rinse of the salt and dry the vegetables very well. 

Heat the oil in a deep pan to 190C. Prepare a plate lined with kitchen paper towel. Working a few at a time, dip the vegetables into the batter, allowing excess batter to drain back into the bowl. Fry until light brown, then drain on the kitchen paper. Move to a serving plate, sprinkle with salt, if you wish, and serve immediately while hot with skordalia.