In a medium bowl, combine the strawberries and blueberries with 1 teaspoon of sugar and toss to combine. Set aside.
On a lightly floured surface, roll out the puff pastry to a 10x12-inch rectangle. Cut a ½-inch strip off each side of the rectangle. Lay the strips on top of each of the edges to create a rim. Transfer the pastry to a baking sheet; prick the center of the pastry all over with a fork. Bake for 20 minutes; let cool.
In a medium bowl, use an electric mixer to combine the cream cheese, sugar and heavy cream. Spread this mixture on the cooled crust. Top with the berries.
Garnish with a few fresh mint leaves, if desired. Serve immediately.