Preheat your oven to 425°F (220°C).

In a large mixing bowl, combine light brown sugar, cornstarch, and cinnamon. Toss sliced peaches in the bowl until well-coated.

Line a round baking sheet with parchment paper.

Unroll the puff pastry on the parchment paper, rolling it out to a 12-inch circle.

Arrange the peach slices in the center of the pastry, leaving a 2-inch border.

Fold the edges of the puff pastry over the peach filling, pleating as you go.

Brush the exposed pastry edges with beaten egg and sprinkle with decorative sugar.

Bake for 25 minutes until golden brown.

Let cool for a few minutes before serving. Dust with powdered sugar if desired.