1. With a sharp knife, slice esrogim into thin, even rounds about 1/8-inch thick. Try to slice evenly and take care to keep as much of the flesh intact as possible. Discard ends, carefully remove and discard any seeds.
2. Bring one and three-fourth cups of water and one cup sugar to a boil, stirring until the sugar is dissolved.
3. Add sliced esrogim and bring back to a gentle boil. Reduce heat as low as possible to maintain a gentle simmer, and simmer for one hour. Remove pan from heat, cover and let sit at room temperature at least four hours or overnight. Gently drain the fruit and discard the liquid – it is rather bitter.
4. Bring remaining two cups sugar, one and a half cups water, and corn syrup to a boil, stirring until sugar is dissolved. Add etrog slices and bring back to a gentle boil. Reduce heat as low as possible to maintain a gentle simmer for 30 minutes. Remove pot from flame, cover and let sit at room temperature at least four hours or overnight.
5. Remove esrog slices from syrup and spread out evenly on a wire rack set over a baking sheet. Allow to dry for at least one hour or overnight.