Cut the apple in half, then slice four 1/8" slices. Make a small diagonal slit in each slice using the tip of a knife. Save the rest of the apple for later.
On a small plate, mix sugar and cinnamon. Wet the rims of champagne flutes, then dip them in the cinnamon-sugar mixture.
Fill each flute a quarter full with apple cider, then top off with Champagne or Prosecco. Garnish each with an apple slice.