In a medium bowl, beat the egg with 1 tbsp. sugar. Add the milk, flour, cornstarch, baking powder, and the zest of half a lemon. Stir to fully combine and set the batter aside to rest for 1 hour.
Slice the apples horizontally, leaving a hole in the center of each slice. Set slices aside and sprinkle with a few drops of lemon juice.
Heat the oil to 350° in a large skillet.
Using tongs, dip the apple slices into the batter, and fry them until golden, between 1 and 2 minutes on each side. Remove them to a paper towel-lined plate to drain and cool.
Arrange the apple fritters on a serving plate and sprinkle with powdered sugar. You may also garnish with whipped cream or your favorite custard.