Carefully unroll the crust onto a piece of parchment paper, then use a rolling pin to roll out the crust to a twelve-inch circle. Dust the rolling pin with flour if the dough starts to stick.
Lift the sides of the parchment paper and place the crust and the parchment paper on a rimmed half-sheet pan.
Juice half a lemon into a medium mixing bowl. Add vanilla extract to lemon juice.
Next, whisk the tapioca flour into the lemon juice mixture until there are no lumps.
Then, slowly whisk in the sugars and cinnamon, starting with the granulated sugar and ending with the cinnamon.
Pour lemon mixture over sliced peaches into a large mixing bowl. Use a spatula to stir peaches to coat with the lemon mixture gently.
Arrange the peaches on the crust in a circle about two inches from the edge.
Next, fill in the remaining center of the circle. Pour any juices left in the bowl over the peaches.
To form the top crust start folding the outer edge of the crust over the peaches, crimping when necessary.
Put small pieces of butter on the peaches, then brush the egg wash on the crust, and sprinkle sugar on the crust.
Bake: Transfer the sheet pan to a preheated oven and allow to bake for 25 - 30 minutes.
After 25 minutes, check if the galette's crust is golden brown. If not, bake for another 5 minutes or until it turns golden brown.
Allow the galette to rest for five minutes before serving.