Combine ingredients in a blender and puree until smooth.

Pour mixture into a square metal pan and place in freezer for exactly 1 hour (make sure to set the timer so you don't forget, or you'll be left with a tough-to-break block of would-be granita).

Remove pan from freezer and stir slush, mashing whole chunks with the back of a fork.

Cover and freeze for another 2 hours at least until mixture is firm, then remove from freezer and with fork tines, stir vigorously until a cool, soft slush forms.

(At this point, you can re-cover the mix and keep in the freezer for up to 3 days.

) Scoop granitas into glasses and serve with a lime garnish for fanciness.