Prepare the pie dough as instructed. Shape the dough into a disk, wrap in plastic wrap, and refrigerate while you prepare the filling.
Save the extra tablespoon of milk and sugar for when you assemble the galette.
Peel the apples and cut them in half. Cut out the apple cores, keeping the halves intact, so that you have 6 apple halves. Thinly slice the apples, about 1/8 inch thick.
In a bowl, combine the granulated sugar, brown sugar, 2 tablespoons flour and the spices.
Save the third tablespoon of flour for sprinkling over the dough when you assemble the galette.
Assemble and Bake
Line a baking sheet with parchment paper. Roll out the dough to about 11 inches in diameter, and place the dough on the paper. Sprinkle the remaining tablespoon of flour over the dough, leaving a 1 1/2 inch border around the edges.
Fan out the apple slices and arrange them over the dough. Sprinkle the apples with the sugar/flour/spice mixture.
Fold up the edges of the dough, so that it covers the apples around the edges by about 1 1/2 inches, with the center open.
Brush the dough with the 1 tablespoon of milk, using it to help seal the creases of dough together. Sprinkle with the remaining 1 tablespoon granulated sugar. Place the galette in the refrigerator to chill while you preheat the oven.
Preheat the oven to 375 F, and position a rack in the center of the oven. Bake the galette for about 50-55 minutes, until the crust is golden brown and crisp, and the apples are soft and the filling is bubbling slightly and thickened.
Cool the galette for at least 30 minutes, then serve warm, or at room temperature.
Store the galette, loosely covered, at room temperature for up to 1 day, or refrigerated for up to 3 days.Note that the topping on the apples may still be a bit floury when the galette is just baked, but as the galette cools, the topping will melt into the apples.To reheat, warm the galette in a toaster oven for several minutes until warm.