Prepare the fruit filling: In a large mixing bowl, combine the blueberries and sliced peaches. Add sugar, cornstarch, lemon juice, vanilla extract, cinnamon (if using), and a pinch of salt. Gently toss everything together until the fruit is evenly coated and slightly glossy. Let it sit for 10–15 minutes so the juices start to release and the mixture thickens slightly.

Prepare the dough: Roll out the puff pastry or pie dough on a lightly floured surface. Cut it into 6 even circles or rustic squares, depending on your preference. Place each piece on a baking sheet lined with parchment paper, making sure there is enough space between them for folding and baking.

Assemble the galettes: Spoon a generous amount of the blueberry peach filling into the center of each dough piece, leaving about a 1–2 inch border around the edges. Carefully fold the edges over the filling, overlapping slightly as you go, creating a rustic open-faced tart shape.

Brush and finish: In a small bowl, whisk the egg with milk or water to create an egg wash. Brush the folded edges of each galette lightly to help them turn golden and shiny while baking. Sprinkle a little sugar over the crust for extra texture and sweetness.

Bake the galettes: Preheat the oven to 375°F (190°C). Bake for 35–40 minutes, or until the crust is deeply golden and crisp and the fruit filling is bubbling and slightly thickened. Rotate the tray halfway through baking for even browning.

Cool and serve: Remove from the oven and let the galettes cool for at least 10–15 minutes so the filling can set slightly. Serve warm or at room temperature, optionally with vanilla ice cream or whipped cream for extra indulgence.