Pulse flour, sugar, and salt in a food processor until combined, or whisk together in a large bowl. Add cold butter and pulse until the mixture resembles coarse meal with some pea-sized pieces of butter remaining.

Gradually add ice water, pulsing until the dough just comes together; avoid overmixing. Form the dough into a disk, wrap it in plastic, and chill for at least 30 minutes.

In a large bowl, gently toss sliced peaches with granulated sugar, cornstarch, lemon juice, vanilla extract, and cinnamon until the peaches are evenly coated.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. On a lightly floured surface, roll out the chilled dough into a 12-inch circle, about 1/8 inch thick, then carefully transfer it to the prepared baking sheet.

Arrange the peach mixture in the center of the dough, leaving a 2-inch border. Fold the edges of the dough up and over the peaches, pleating as needed.

Brush the crust with beaten egg and sprinkle with turbinado sugar for a golden, slightly crunchy finish.

Bake for 40-45 minutes, or until the crust is golden brown and the peach filling is bubbly. Let the galette cool slightly on a wire rack before serving.