Preheat oven to 350°F. Line a large baking sheet with parchment paper.
Press the two pie crusts together to form a ball. Roll it out into a 12-inch circle on a clean surface. Transfer to the prepared baking sheet.
In a large bowl, toss apple slices with granulated sugar, flour, and cinnamon. Spread the apple mixture evenly in the center of the crust, leaving a 2-inch border around the edges.
Shape the galette: Fold the edges of the crust up and over the apples, working your way around the circle.
Make the topping: In a small bowl, combine flour, brown sugar, butter, and cinnamon with a fork until crumbly. Sprinkle evenly over the apples.
Bake: Place the galette in the oven and bake for 40-50 minutes, or until the apples are tender and the crust is golden brown.
Cool and serve: Let the galette cool for at least 10 minutes. Serve warm with caramel sauce, ice cream, or whipped cream, if desired.