In a saucepan over low heat, combine water and sugar, stirring until the sugar completely dissolves to create a simple syrup.
Remove the syrup from heat and allow it to cool completely.
Once the syrup is cool, stir in the Limoncello, lemon juice, and lemon zest.
Pour the mixture into an ice cream maker and freeze according to the manufacturer’s instructions.
If you don’t have an ice cream maker, pour the mixture into a shallow dish and freeze, stirring every 30 minutes until it reaches sorbet consistency.
Serve the sorbet in chilled bowls or glasses, garnished with a lemon slice or fresh mint if desired.