In a large bowl, combine the ingredients for the crust: the flour, sugar, cornmeal, and salt.
Add butter and with a pastry cutter, blend in the butter until the mixture resembles coarse meal.
Pour the buttermilk (or milk/vinegar mixture) into the bowl and stir until the dough begins to stick together.
Remove the dough ball and adhere any remaining pieces of dough to it, then wrap in plastic wrap.
Refrigerate for at least 45 minutes before rolling out.
Preheat oven to 350 degrees F. when ready to bake the galette.
For the filling, in a large bowl, combine blueberries, peaches, sugar, corn starch, lemon juice and zest, and nutmeg.
Gently mix everything together so that you don’t crush the blueberries. If you want to you can, but I wanted to keep the peaches as white as possible.
Place a piece of parchment on a baking sheet. I place it on my round pizza stone because I found that it’s the perfect size and bakes it quite nicely.
Remove the dough from the refrigerator and roll it out, starting from the center, into a circle (or as close as you can get to a circle) 14 inches across.
Place the filling contents into the center of the circle and then gently spread around the circle to within two inches of the edge.
Fold the edges of the dough up over the filling, overlapping the dough if necessary, and gently pressing the creases to ensure that creases are sealed.
Lightly brush the edge of the dough with the beaten egg and sprinkle with turbinado sugar. (You will have egg left over.)
Bake for 1 hour, until the crust is golden brown and the filling is bubbly. Transfer baking sheet to a wire rack and allow to cool for 20 minutes before serving.