Prepare an ice-water bath; set aside. Using a mandoline or sharp knife, cut lemon into 12 paper-thin slices; discard seeds and ends of rind.
Bring a medium saucepan of water to a rolling boil. Remove from heat, and add lemon slices; stir until softened, about 1 minute. Drain, and immediately plunge slices into ice-water bath. Drain.
Bring sugar and 1 cup water to a boil in a medium skillet, swirling to dissolve sugar. When liquid is clear and bubbling, reduce heat to medium-low. Add lemon slices, arranging them in one layer with tongs. Simmer (do not let boil) until rinds are translucent, about 1 hour.
Transfer to a baking sheet lined with parchment. Let stand until ready to serve. Lemon slices can be stored in an airtight container at room temperature up to 1 day.