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

In a mixing bowl, combine mixed berries, lemon juice, lemon zest, maple syrup, and cornstarch. Stir until evenly coated.

Unroll the puff pastry on a lightly floured surface and roll it out to about 1/4 inch thickness. Cut into 4 equal rectangles.

Transfer the pastry rectangles to the baking sheet and spoon the berry mixture onto the center of each, leaving a generous border.

Fold the edges of the pastry over the filling and pinch the corners to seal.

Whisk an egg and use it to brush the edges of the tarts for a golden finish.

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

Let cool for 5 minutes and serve warm, preferably with a scoop of vanilla ice cream.