Combine the flour, sugar, and no-salt substitute 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, about 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, whisk the egg yolk and milk together 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 preheat the oven to 350°F. Line a large, rimmed baking sheet with parchment paper.
In a medium bowl, toss the peaches and blueberries with the brown sugar, flour, cinnamon and nutmeg.
Lightly flour a large work surface and roll out the dough to a 12 to 13- inch round. Transfer to the prepared baking sheet.
Mound the fruit in the center of dough, leaving a 2" border.
Carefully grasp one edge of the dough and fold up 2" over fruit. Repeat around circumference of galette, overlapping dough every 2"; firmly pinch pleated dough to secure, but do not press dough into fruit.
Arrange the fruit in the center of the dough, leaving about 1 to 1 1⁄2” of space around the perimeter of the dough. Brush the crust with the egg wash. Sprinkle the turbinado 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 with dollop of crème fraîche or vanilla bean ice cream.