Add the flour, cornstarch, baking powder, sugar and salt to a medium sized mixing bowl and whisk until the ingredients are combined.
In a separate bowl, whisk the eggs, lemon zest, and milk together.
Then, add the wet ingredients to the dry and whisk until the mixture is homogeneous. Allow the batter to rest for fifteen minutes.
Meanwhile, set up a frying station. Set up a cooling rack and add about 2" of olive oil to a heavy bottomed pot or dutch oven and heat to 375°F.
Dip an apple slice in the batter and gently place in the olive oil. Be careful not to over crowd the pan because the oil temperature will decrease. Fry the fritters until they are golden, about 45-60 seconds on each side.
When the fritters are done, remove with a slotted spoon and allow to drain on a baking sheet with a cooling rack.
Sprinkle with confectioners sugar and/or cinnamon and serve. Frisceu are best served hot from the pan.