1. Combine sugar and 1/2 cup water in a small saucepan, and cook over medium heat, stirring often, 3 minutes or until sugar dissolves. Remove pan from heat; stir in lemon zest. Let cool to room temperature, about 30 minutes.
2. Strain mixture through a fine wire-mesh sieve into a blender, pressing solids with a wooden spoon to extract liquid. Discard solids.
3. Add ice cubes, lemon juice, and limoncello, if desired, to blender and process until smooth and slushy. Garnish, if desired. Serve immediately.