Cut the lemon into 1/8-inch (3 mm) thick slices. Remove the seeds. Place in a small saucepan.
Cover with cold water. Bring to a boil and let simmer for 2 minutes. Drain the lemon slices and set aside. Discard the cooking water.
In the same saucepan, bring the sugar, water and limoncello to a boil until the sugar has dissolved. Add the lemon slices and let simmer over medium heat for about 10 minutes or until tender and translucent. Let cool completely in the syrup. When ready to serve, drain the lemon slices.