On a floured surface, roll out pie dough until its at least 8 inches across. Let pie dough chill in the fridge while making the filling.
For the filling, add peach slices, sugar, cinnamon, nutmeg and cornstarch to a glass bowl and mix until well combined.
Remove chilled pie dough from fridge and add peach slices starting from the center of the dough and building outwards. Leave about 2 inches of dough on the edges in order to fold over peaches for the crust.
Once the edges have been folded over the peaches, add some cubed butter on top of the peaches and paint egg wash on the crust. Finish the crust with a couple sprinkles of sugar and put in the oven for 35 – 40 minutes.
Once crust is golden and crispy, remove from the oven and gently move galette to a rack to cool. Serve while still warm with ice cream or whipped cream.