• To make the pastry dough, combine the gluten-free flour, salt, and sugar in a large bowl. Add the cold butter and use a pastry blender or your fingers to mix until the mixture resembles coarse crumbs. Gradually add ice water, stirring until the dough begins to come together. Form into a disc, wrap in plastic wrap, and chill for at least 1 hour.
• Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
• For the filling, melt the butter in a large skillet over medium heat. Add the sliced apples, brown sugar, cinnamon, lemon juice, vanilla extract, and a pinch of salt. Cook, stirring occasionally, until the apples are tender and caramelized, about 10 minutes. Remove from heat and let cool slightly.
• On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Transfer to the prepared baking sheet. Arrange the caramelized apple mixture in the center of the dough, leaving a 2-inch border. Fold the edges of the dough over the apples, pleating as needed.
• Brush the dough with the beaten egg and sprinkle the granulated sugar over the apples and the dough. Bake in the preheated oven for 35 minutes, or until the crust is golden brown and the filling is bubbly. Let the galette cool slightly before slicing and serving.

Tip:

• For a beautifully flaky crust, keep the butter and water as cold as possible when making the dough.