To make the pistou, crush the basil and garlic with a mortar and pestle to a fine paste. Transfer to a bowl and add the cheese and lemon zest. Gradually stir in the oil until combined and place in a serving dish.
For the fritters, place the grated zucchini in a strainer and sprinkle with plenty of salt. Stand for 1 hour and rinse well. Dry on paper towels.
Sift flour and make well in center and add the egg yolk and oil. Measure in 5 T water. Beat the yolk and oil and gradually incorporate the flour and water to make a smooth batter. Season with salt and pepper. Let rest for 30 minutes.
Stir the zucchini into the batter. Beat the egg whites stiff and fold into the batter.
Heat 1/2" of oil in pan and add spoonfuls of batter and fry for 2 minutes until golden. Drain the fritters on paper towels and keep warm. Serve with pistou sauce.