Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

In a bowl, combine mixed berries, lemon juice, lemon zest, maple syrup, and cornstarch. Stir to coat the berries.

Roll out the thawed puff pastry on a floured surface and cut into four rectangles.

Place mixed berry filling in the center of each pastry rectangle and fold edges over to create pockets.

Brush edges with beaten egg to create a golden color.

Bake for 20-25 minutes until golden brown and puffed up.

Allow to cool slightly before serving warm, possibly with vanilla ice cream.