Preheat your oven to 350°.
Line a baking sheet with parchment paper and set aside.
In a mixing bowl, gently toss together the sliced peaches, sugar, flour, and cinnamon until evenly coated.
Place the pie crust dough in the center of your lined baking sheet.
Spoon the peach filling into the center of the pie crust dough and fold the edges of the pie dough inward, leaving a large hole in the center to allow steam to escape while baking.
Use a pastry brush to lightly brush the edges of the galette with the beaten egg.
Sprinkle the egg-washed pie dough generously with sugar, then place the galette into the oven.
Bake for 25-30 minutes, or until the edges of the galette are golden brown and the center is set.
Allow to cool for at least 10 minutes, then serve warm or room temperature with a scoop of vanilla ice cream or a dollop of whipped cream, if desired.