Low Carb Gratin of Fresh Berries

Ingredients:1/2 pint fresh raspberries1/2 pint fresh blackberries1/2 pint fresh strawberries, cut into quarters1/2 pint fresh blueberries1/4 cup plus 1 tablespoon sugar substitute (recommended: Splenda)1 cup sour cream1 tablespoon brown sugar substitute (recommended: Sugar Twin)Fresh mint, for garnish