Preheat the oven to 425°
In a medium bowl, combine peaches and blueberries. Add sugar, salt, butter, cornstarch, lemon zest, lemon juice, and thyme and stir to incorporate. Place in the refrigerator for 20 minutes.
Roll out the chilled galette dough (also see below) onto a lightly floured piece of parchment paper until the dough is about 12" in diameter and ⅙" thick Lift and gently place the parchment paper onto a rimmed baking sheet.
Using a slotted spoon, add the fruit mixture to the middle of the galette dough. Add as much of the remaining syrup as you like, but keep an eye on how much it's running toward the edges of the galette. Ideally you'd like about a 2" border of dough around the filling.
Fold up the edges of the galette dough up and over the fruit mixture, working your way around the galette. Pinch closed any cracks in the dough.
Using a pastry brush, wipe the edges of the galette dough with the egg wash and then sprinkle the edges with turbinado sugar.
Bake at 425° for 15 minutes, then reduce oven heat to 350° and bake until crust is golden brown, an additional 25-30 minutes.
Cool on a wire rack for 30 minutes and serve with a scoop of ice cream!