Preparation

Make the dough

• Combine the flour, sugar, and salt in a stand mixer fitted with a paddle attachment at low speed. Add the butter to the flour. Mix until the flour is no longer white and holds together when you clump it with your fingers, 1 to 2 minutes. If there are still lumps of butter larger than the size of peas, break them up with your fingers.
• In a small bowl, beat the egg yolk and milk, and add to the flour mixture. Mix on low speed just until the dough comes together, about 15 seconds; the dough will be somewhat soft. Turn the dough out onto a sheet of plastic wrap, press it into a flat disk, wrap it in the plastic, and let it rest in the refrigerator for 15 to 20 minutes before rolling out.
• Meanwhile, position a rack in the center of the oven and heat the oven to 350°F. Line a large rimmed baking sheet with parchment paper.

Make the filling & roll out the dough

• In a medium bowl, toss the peaches and blueberries with the muscovado sugar, flour, cinnamon, and salt.
• Lightly flour a large work surface and roll out the dough to a 12- to 13-inch round. Transfer to the prepared baking sheet. Arrange the fruit in the center of the dough, leaving about 1-1/2 inches of space around the perimeter of the dough empty. Fold the outside edge of the dough over the fruit, making occasional pleats. Brush the crust with the egg. Sprinkle the demerara sugar evenly over the dough and fruit.
• Bake the galette until the crust turns a light brown and the filling bubbles, about 50 minutes. Let cool for 10 minutes then cut into wedges and serve warm.