1. In a small saucepan, combine ¾ cup of the water, ¼ cup of the mint leaves, the sugar and lemonzest. Bring to a simmer over medium-high heat and cook until the sugar has dissolved, 2 minutes.
2. Let cool completely, then strain into a blender.
3. To the blender, add the remaining ingredients and blend until a smooth slush forms.
4. Divide between glasses, garnish with mint sprigs and serve.