Preheat your oven to 400°F and remove your pie dough (or puff pastry dough) from the fridge.
Lay a piece of parchment paper on a clean surface and dust with a little flour. Then roll your dough out into a somewhat circular shape that's about 12 inches in diameter and ⅛-inch thick and transfer it (with the parchment paper) to your baking sheet. Place in the fridge while you make the filling.
Make the filling by combining the peach slices, sugar, lemon zest and juice, tapioca flour, vanilla extract, ground cinnamon, and a pinch of salt in a bowl.
Pour the filling into the center of the rolled-out dough and scatter the peach slices around, layering them as needed so that you have a 1 ½ to 2-inch space around the edges.
Fold small sections of the dough over your filling, slightly overlapping the previous one until all the edges are folded up.
Make the egg wash by whisking together the egg and cold water in a small bowl. Brush it on the folded-up edges and then sprinkle them generously with turbinado sugar.
Bake for 35-40 minutes or until the crust is lightly golden brown. Let cool slightly and serve with a scoop of vanilla ice cream if desired