Preheat your oven to 400 degrees Fahrenheit. If your oven runs hot, set it to 375 degrees Fahrenheit. (Use an oven thermometer to ensure the correct temperature)
Remove the chilled pie dough from the fridge and let it sit for about 10 minutes before rolling. Roll out the ball of dough into a large circle using a rolling pin. Add a generous amount of flour to the rolling surface and to the rolling pin, to prevent sticking and cracking. Peel the rolled out dough off the counter surface gently by rolling it backwards onto the rolling pin.
Transfer it to baking sheet lined with parchment paper
Add the blueberry filling and pile it into the middle of the dough, leaving a large edge.
Fold the edges of the dough up onto the pile of blueberry filling, creating the edge crust. pinch the pieces you fold up into each other to seal them shut (so the filling can't escape).
Beat the egg and 1 tablespoon of water and brush it lightly onto the unbaked crust. Use a light hand while brushing - too much egg wash creates an eggy texture after baking.
Dust the crust with the coarse sugar and bake at 400 degrees Fahrenheit for about 40 to 45 minutes or until the filling is bubbly and the crust is golden brown.
Let the galette cool to room temperature before slicing and serving with whipped cream or ice cream.