1. Halve lemons crosswise and juice, if desired, reserving juice for another use. Cut each half into 4 equal pieces. Using a small sharp knife, remove the flesh from skin; if flesh has not been juiced, reserve for another use. Cut skin (pith and peel) into 1/2-inch pieces; set aside.

2. Fill a large saucepan with water and bring to a boil over high heat; reduce to a simmer and add lemon skin; let cook until skin offers no resistance when chewed, about 45 minutes. Drain and set aside.

3. Fill a large saucepan with 3 1/4 cups water. Add sugar and cream of tartar. Place over medium high heat and cook until sugar has dissolved. Add lemon skin to saucepan and bring to a simmer; let cook, stirring occasionally, until mixture reaches 230 degrees on a candy thermometer, 50 to 55 minutes.

4. Drain lemon skin and place on a wire rack set over a parchment paper-lined rimmed baking sheet; let dry until less sticky, about 2 days. Gently toss in sugar to coat. Store in an airtight container in a cool dry place; do not refrigerate.