Separate the eggs and refrigerate the egg whites. Whisk the flour, milk and egg yolks together until smooth. Cover and let stand for about 20 minutes.
Peel the apples, remove the cores and cut into 1 cm (approximately 3/8 inch) thick slices. Mix 2 tablespoons of sugar with the grated lemon zest in a shallow dish.
Melt the butter and stir into the batter. Beat the egg whites until stiff and fold into the batter. Heat the frying oil in a large saucepan to 170°C (approximately 340°F). Dip the apple slices in the lemon sugar then dip into the batter and carefully drop into the hot oil. Fry until golden brown.
Drain the apple fritters on paper towels. Mix the remaining sugar with the cinnamon and toss the apple fritters in the cinnamon sugar while still hot. Serve immediately.