This recipe is used in our Lemon-Thyme Pound Wedding Cake with Meringue Buttercream.
In a large bowl, prepare an ice-water bath; set aside. Using a mandolin or sharp knife, cut each lemon into 12 paper-thin slices; discard seeds and ends of rind.
Bring a medium saucepan of water to a rolling boil. Remove from heat, and add lemon slices; stir until softened, about 1 minute. Drain, and immediately plunge slices into ice-water bath; drain.
Set a wire rack over a sheet pan; set aside. Bring the sugar and 2 cups of water to a boil in a large skillet, swirling to dissolve sugar. When liquid is clear and bubbling, reduce heat to medium-low. Add lemon slices, arranging them in one layer with tongs. Simmer (do not let boil) until rinds are translucent, about 1 hour. Transfer slices to the sheet pan with the wire rack; let slices drain and cool.