Step 1

To make the pastry, combine the flours, granulated sugar, and salt in a food processor and pulse until blended. Scatter the butter over the flour mixture and pulse until it looks like coarse sand. (Alternatively, combine the flour, sugar, and salt in a bowl and work the mixture with a pastry blender until it has a sand-like texture.) In a small bowl or cup, whisk together the 1 egg and the milk until blended, pour into the food processor or bowl all at once, and pulse or gently mix by hand until the dough forms a rough clump. Do not overwork the dough to avoid toughness.

Step 2

Lightly flour a work surface. Transfer the dough to the floured surface, form it into a ball, and then flatten into a disk 1 to 1 1/2 inches thick. Don't knead it! Wrap it in plastic wrap and refrigerate for at least 1 1/2 to 2 hours or for up to 1 day.

Step 3

When the dough is well chilled, preheat the oven to 375°F.

Step 4

Remove the dough from the refrigerator and let it warm at room temperature for about 10 minutes. Meanwhile, make the filling. In a small bowl, stir together the granulated sugar, the flour, and the cinnamon, mixing well. Place the peaches in a bowl, sprinkle the sugar mixture over the top, and toss gently. (If the peaches are tart, you may want to add a little more sugar.)

Step 5

Lay a large sheet of parchment paper on a work surface and dust the parchment with flour. Place the dough on the floured parchment and roll it out into a round about 11 inches in diameter and 1/8 inch thick. Transfer the pastry with the parchment to a rimmed or rimless baking sheet large enough to accommodate the pastry round comfortably. Starting 1 to 1 1/2 inches from the edge of the round, arrange the peach wedges in a single layer in concentric circles, continuing until you reach the center of the round. Fold the outer edges of the pastry over the peaches, creating overlapping folds as you work around the perimeter. If your peaches are extra juicy, leave the extra juice in the bowl so that the filling does not overflow in the oven. Brush the pastry border with 1 tablespoon beaten egg and then sprinkle lightly with the turbinado sugar.

Step 6

Bake for 50 to 60 minutes, until the crust is golden and the filling is bubbling. If your galette has a lot of juice, you may want to place a second pan on the lowest rack in the oven under the baking sheet to catch any juices that overflow.

Step 7

Let the galette cool on the baking sheet on a wire rack. Cut into wedges to serve.