Remove rind from watermelon, cut into 2-inch chunks.
In a blender, add the watermelon, raspberries, sweetener, and lemon juice. Purée in batches until smooth.
Strain through a fine sieve into a large baking pan, pressing down to extract as much juice as possible.
With a spatula smooth mixture. Cover.
Freeze for up to 4 hours or overnight.
Stir or scrape mixture with a fork after an hour to break up the forming ice crystals, until firm but not completely frozen.
Return to the freezer. Repeat in another hour.
It should be ready to serve between the fourth or six hour. The freezing time will depend on the size of the container you use.
To serve, scrape with a fork and spoon into small chilled glasses.
Garnish with the mint and serve.
Notes
Chefs Note: Be sure to CHILL glasses!! For a time saver and short cut, feel free to purchase pre cut watermelon in your grocer’s produce section. Give raspberries a rinse just before using, no sooner!