Using a mandolin, slice lemons into paper-thin rounds; remove seeds and discard.

In a medium bowl, combine lemon slices and sugar. Let stand until sugar is dissolved, about 30 minutes.

In a medium saucepan, bring lemon-sugar mixture and 2 cups water to a boil over high heat. Reduce heat to medium-low, and simmer, stirring occasionally, until translucent, about 30 minutes, being careful not to let mixture caramelize.

Remove from heat, and let cool slightly. Strain mixture, reserving lemon slices. Place lemon slices on parchment paper; let dry at room temperature. (Lemons will be sticky.) Cover and freeze for up to 1 week.