Method:
In a blender, combine orange juice, water, agave nectar, sugar, zest and half of the strawberries and raspberries and purée until almost smooth. Add remaining berries and purée until completely smooth. Taste and adjust the sweetness with additional agave nectar, if needed.
Pour berry mixture into a 9- x 13-inch nonstick metal baking pan and freeze until edges become icy and center is slushy, about 1 hour. Using a fork, stir the icy edges into the middle of the pan, repeating this step every 30 minutes until granita is completely frozen, about 3 hours.
Using a fork, scrape granita down the length of the pan, forming flaky ice crystals. Spoon into bowls, garnish with mint and serve or cover tightly and freeze for up to 24 hours.