Step 1

Place sugar and 3/4 cup water in a small saucepan. Bring to a boil over medium heat, stirring occasionally. When sugar has dissolved, remove pan from heat, and stir in creme de menthe, if using. Chill the syrup.

Step 2

Remove seeds from watermelon flesh. Place flesh in the bowl of a food processor; process until liquefied, about 2 minutes. Add puree to chilled syrup; stir to combine.

Step 3

Freeze sorbet in an ice-cream maker according to the manufacturer's instructions. Store, frozen, in an airtight container.