Make the pie dough. Once the dough is made, wrap it in plastic wrap and refrigerate it for at least 1 hour, so it has proper time to chill.
Remove sourness from apricots (Optional). In a medium saucepan, bring 3 cups of water and 1 cup of granulated sugar to a boil. Once boiling, add the pitted apricots and let them simmer for about 5 minutes. Use a slotted spoon to remove them from the syrup. Let the apricots cool for 5-10 minutes before proceeding with the recipe.
Preheat the oven to 400ºF/200ºC.
Prepare the filling. In a medium bowl, combine sliced apricots, brown sugar, tapioca flour, vanilla extract, and ground cinnamon. Use a spoon to gently mix the fruit until evenly coated.
Roll out the dough on a lightly floured surface. Lay a piece of parchment paper on a clean surface and dust with a little flour. Roll the dough out into a somewhat circular shape that's about 12-inches in diameter and ⅛-inch thick. Transfer it (with the parchment paper) to your baking sheet.
Add the filling. Pour the filling ingredients into the center of the rolled-out dough and scatter the apricot slices around, layering them as needed so that you have a 1 ½ to 2-inch space around the edges.
Fold 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. In a small bowl, whisk together one whole egg and cold water in a small bowl. Use a pastry brush to brush the egg wash on the folded-up edges of the dough, then sprinkle them generously sprinkle with turbinado sugar.
Bake galette. Bake for 35-40 minutes in the middle of the oven or until the crust is a deep golden brown color. Let cool slightly and serve with a scoop of vanilla ice cream, coconut whipped cream, or pistachio cream if desired.