Cut the lemon crosswise into thin (just under 1/8-inch thick) rounds. Combine the sugar with 2/3 cup water in a medium skillet and bring to a bare simmer over medium-low heat. Add the lemon slices and cook at a bare simmer until the rinds are almost completely translucent, about 20 minutes. Remove from the heat and let stand in the syrup until cool. Transfer the lemon slices to a cooling rack set over a baking sheet and allow to dry for 3 hours at room temperature.
Strain the remaining cooled simple syrup into an airtight container or jar and reserve for another use (simple syrup keeps in the refrigerator for up to 2 weeks).
Pour 1 1/2 tablespoons limoncello into each of 6 Champagne flutes, then fill with the prosecco. Garnish each flute with a candied lemon slice and serve.