In a saucepan over medium heat, combine diced freestone peaches, granulated sugar, ground cinnamon, and 1 tablespoon of water. Stir gently for about 5–8 minutes until the peaches soften and release their juices, creating a lovely syrup.

In a small bowl, mix the remaining tablespoon of water with cornstarch until smooth. Stir this mixture into the simmering peach blend, cooking for an additional 2–3 minutes until it thickens.

While the peach filling cools, roll out your cold, store-bought puff pastry on a lightly floured surface into a 15x15-inch square.

Take each pastry square and place about 1 tablespoon of your cooled peach filling in the center. Brush the edges lightly with beaten egg to create a seal.

Preheat your oven to 400°F (200°C). Before baking, chill the filled turnovers by refrigerating them for 15 minutes.

Once chilled, transfer the turnovers to the preheated oven. Cut three small slits on the top of each turnover to allow steam to escape. Bake at 400°F for 15 minutes, then reduce the temperature to 350°F and continue baking for an additional 10-15 minutes until golden brown.

Allow the turnovers to cool on a wire rack for about 5 minutes before serving.