In a large bowl, mix together the flour, sugar, and salt. Drop in the cold butter cubes and toss them through the flour so they are well coated.

Use your fingertips to pinch and rub the butter into the dry ingredients until the mixture looks like wet sand with no large butter chunks. A food processor works well for this too.

Create a well in the center and add the egg and cold water. Stir with a spatula until the dough begins to clump together, then bring it together with your hands. Be careful not to overwork the dough or it will become tough and difficult to roll.

Press the dough into a disc, wrap it in plastic, and refrigerate for at least 30 minutes. If the dough becomes very firm, let it sit at room temperature for a short while. If the edges are hard but the center is soft, knead it briefly on an unfloured surface.

Dust your work surface with flour and roll the dough out to about 1/8 inch thick. Avoid rolling it thinner, or the fruit juices may cause the crust to break in the oven. Move the dough and add flour underneath as needed to keep it from sticking, aiming for a circular shape. Rolling between two sheets of parchment paper is a good alternative.

Lay the dough onto a baking sheet lined with parchment paper or a silicone mat. If you rolled the dough between parchment sheets, simply peel off the top sheet and transfer the dough with the bottom parchment directly onto the tray.

You can keep the edges rough and rustic, or use a large plate as a guide and cut around it with a knife. Make sure the dough is already on the baking sheet before adding the peaches, because the filled galette cannot be safely transferred afterward.

Slice the peaches into pieces about 1/8 inch thick. To prep them, cut around the natural seam of each peach, twist both halves to separate them from the pit, then dislodge the pit and slice as needed.

In a bowl, gently toss the peach slices with the sugar, cinnamon, cornstarch, and melted butter until everything is evenly coated. Set the raspberries aside for now.

Pile the peach slices onto the dough in a circular pattern, starting from the outside edge and working toward the center. Leave a border of about 2 inches around the filling. Put the thinnest slices in the middle so they can bend and tuck easily. Sprinkle the raspberries over the top of the peaches.

Fold the exposed dough edges inward over the filling, pleating as you go. Brush the folded dough with milk.

Bake the galette in a 350°F oven for 30 minutes, or until the crust is golden brown and the filling is bubbling. A little juice seeping out is completely normal.

Let the galette cool slightly before cutting and serving. It is wonderful with a scoop of vanilla ice cream.