Mix all ingredients for batter (flour, salt, granulated sugar, milk, lightly beaten eggs, lemon zest, and melted unsalted butter) in a bowl until smooth. Cover with plastic wrap and set aside to rest for 30 minutes.While batter is resting core and slice apples 1 cm thick. Set aside. Start heating oil.Dip apple slices in batter, then fry until golden on each side.
Drain the apples well on double lined paper towels. Then sprinkle the cinnamon sugar (brown sugar and cinnamon combined) on the slices.