Preheat oven to 350 degrees F.

Combine white whole wheat flour, Kosher salt, and sugar in a medium bowl and whisk together thoroughly.

Add to dry ingredients, cut-up butter.  Use a pastry blender to mix the dry ingredients with the butter until it has a sand-like texture.

In a small bowl, combine egg with milk until blended, and then pour into flour and butter mixture all at once. Gently mix by hand until the dough begins to clump together. Avoid overmixing  to prevent toughness.

Cover a 12-inch pizza pan with parchment paper cut to fit the pan. Turn dough out onto the middle of the pizza pan; lay on top of the dough, a piece of waxed paper and carefully press or roll the dough, between the parchment paper on the bottom of the dough and waxed paper on top of the dough, into a 12-inch round to fit the pizza pan.

Cover the rolled out dough on the pizza pan with either waxed paper or parchment paper and place in the freezer for 5 minutes.

While the dough is in the freezer, prepare the filling. First, combine 3 tablespoons of white whole wheat flour, 4 tablespoons of sugar, and the ground cinnamon in a small bowl until well mixed.

Peel, pit, and cut up the peaches and place in a medium bowl. Then add the sugar/flour/cinnamon mixture to the peaches and gently mix together thoroughly.

Remove pizza pan from freezer, place peach mixture directly in the middle on top of the dough, leaving a 3-inch border. The dough may need to warm up a bit in order to handle better.

Carefully fold the dough edges up and over the peaches, pleating as you go and leaving about a 5-inch diameter circle of exposed peaches in the center.

Whisk egg white and water in a small bowl and then brush with a pastry brush, over the doughs edges.  Sprinkle the edges lightly with turbinado sugar.

Bake for 50 minutes or until the crust is golden brown and the filling is bubbly.

When galette is done, remove it from the pan and parchment paper carefully and transfer to a wire rack.  Let cool for at least 30 minutes or up to 1 hour before slicing and serving.