Let the Galette Dough sit and come to room temperature. If it's too cold, it will crumble as you fold the edges over, which isn't a problem taste wise, but it will really irritate you since the filling will ooze all over the place.
Mix together the cornstarch, sugar, cinnamon and salt in a large bowl and add the blueberries, lemon zest and juice.
On a large piece of floured parchment paper, roll out the dough about 1/8" thick, 13-14 inches. Try to keep it roughly round.
Place the dough in a rimmed baking sheet. It's easy to just lift up the parchment paper with the dough and place it on the baking sheet.
Arrange the fruit mixture in the center of the dough leaving a couple inches of dough around the edges. Then fold the edge of the dough up over the fruit filling, pinching it to stay in place.
Brush the egg over the edges of the galette and sprinkle the sugar all around the rim.
Bake at 450° until the fruit filling is bubbling and the pastry is golden brown, approx. 30 minutes.
Notes
This is even more amazing when served with some vanilla ice cream!