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

Lightly flour a work surface and roll out the puff pastry into a roughly 10x12-inch rectangle. Keep it chilled as you work.

In a medium bowl, combine the mixed berries, granulated sugar, cornstarch, and vanilla. Toss gently until the fruit is coated. Let sit for 5 minutes for the sugar to draw juices.

Spoon the berry mixture onto one half of the pastry, leaving a 1-inch border all around.

Fold the empty pastry half over the filling to form a pocket. Press the edges together, then crimp with a fork to seal.

Brush the top of the strudel with the beaten egg and score three or four slits in the top for steam to escape.

Bake on the prepared sheet for 25-30 minutes, or until the pastry is puffed and evenly golden.

Remove from the oven and let cool for 10-15 minutes. Dust with powdered sugar before serving.