1. Cover lemon slices with water by 1/2 inch in a medium saucepan. Bring to a boil. Drain. Repeat 3 times. Return slices to pan. Add syrup. Bring mixture to a simmer over medium heat. Reduce heat to medium-low; cook until slices are just translucent, about 20 minutes. Let cool.

2. Refrigerate slices in syrup in an airtight container until ready to use, up to 1 week. Before using, drain slices on paper towels, and then finely chop.