Combine water, sugar and mint leaves in a small sauce pan to make simple syrup. Stir until sugar dissolves. Remove from heat and let sit for 20 minutes.
Puree strawberries, blending in lemon juice and mint from simple syrup.
Combine pureed strawberry mixture, prosecco and simple syrup in a glass baking dish (I used an 8×8). Place in freezer for 4-5 hours scraping the mixture with a fork every hour to break it into pieces.