In a large bowl, whisk together the gluten free flour blend, sugar, xanthan gum and salt.

Add the cubed cold butter and toss it in the dry ingredients. Using your fingertips or a pastry cutter, work the butter into the dry ingredients until it's about the size of walnut halves.

Start adding the cold water, about 1 tablespoon at a time. After each addition, use your fingertips or a fork to toss the mixture around so that the flour gets evenly hydrated. Continue adding the water and tossing the mixture until all the flour has been hydrated and the mixture starts clumping together in places.

Bring the pie dough together into a ball by giving it a gentle knead and pressing it against the sides of the bowl. Wrap the pie dough in plastic wrap/cling film, and chill it in the fridge for at least 30-45 minutes.

On a lightly floured work surface, roll out the chilled pie dough into a long rectangle about ¼ inch (about 5-6mm) thin. First letter fold: Fold one third of the rectangle towards the centre and then fold the other third over it. Rotate the folded pastry by 90 degrees, and roll it out again into a long rectangle. Second letter fold: Fold one third of the rectangle towards the centre and then fold the other third over it. Wrap the finished pie dough tightly in cling film/plastic wrap and chill it in the fridge for at least 30-45 minutes.

Adjust the oven rack to the lower middle position and pre-heat the oven to 400ºF/200ºC. Cut the peaches into about ½-inch (1-1.5cm) thick slices/wedges.

Lightly flour a large sheet of parchment/baking paper and place the chilled pie crust in the centre. Roll out the pie crust into a rough circle, about 13-14 inches (32-36cm) in diameter.

Add the sugar, cornstarch, lemon juice and vanilla to the sliced peaches and toss to combine.

Arrange the peach filling in the centre of the rolled-out pie crust in an even layer, leaving a 2-inch (5cm) border free of any filling. Fold the edges of the pie crust over the filling, overlapping the pastry as necessary.

Drizzle the juices left in the bowl over the filling. Brush the pie crust with the whisked egg, and sprinkle both the crust and the filling generously with granulated sugar.

Slide the assembled galette together with the parchment/baking paper onto the baking sheet. Bake on the lower middle oven rack at 400ºF/200ºC for about 40-45 minutes or until the pie crust is deep golden brown and you can see the juices in the filling visibly bubbling.

Allow the galette to cool until warm before slicing and serving, ideally with a scoop of vanilla ice cream.