Cut the watermelons into large cubes and it them to a food processor. Add the lemon zest and juice and sugar. Puree the watermelon on low speed using the pulse function of the food processor (or the on/off button).
Pour the puree into 9x9 baking pan and freeze for 2 hours. Take the mixture out of the freezer and stir with a fork to create the crystal like texture. Freeze for another 3 hours.
Spoon a half a cup into a dessert glass and decorate with a piece of mint.