These lemon slices go in the filling of our lemon-pistachio tart.
Bring a small pot of water to a boil. Add lemon slices; boil gently until blanched, about 5 minutes. Drain; set slices aside.
Bring sugar and 1 cup water to a boil in a small saucepan until sugar is dissolved. Add blanched lemon slices, and simmer until lemons turn almost translucent, about 30 minutes. Let cool completely in syrup. Store slices in syrup in an airtight container in the refrigerator up 2 days.