Make the Dough: In a large bowl, whisk together the flour and salt. Add the cold, cubed butter and use a pastry blender or your fingers to cut the butter into the flour until the mixture resembles coarse crumbs. Gradually add the ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix.
Chill the Dough: Form the dough into a disc, wrap it in plastic wrap, and chill in the refrigerator for at least 30 minutes. This step is important to relax the gluten and make the dough easier to work with.
Prepare the Filling: While the dough is chilling, prepare the blueberry filling. In a large bowl, combine the blueberries, sugar, cornstarch, lemon juice, and lemon zest. Gently toss to coat the blueberries evenly.
Roll Out the Dough: Preheat your oven to 375°F (190°C). On a lightly floured surface, roll the chilled dough into a 12-inch circle. Transfer the dough to a baking sheet lined with parchment paper.
Assemble the Galette: Pour the blueberry filling into the center of the dough, leaving a 2-inch border. Fold the edges of the dough over the filling, pleating as you go to create a rustic look.
Add Egg Wash (Optional): In a small bowl, whisk together the egg and milk for the egg wash. Brush the egg wash over the folded crust and sprinkle with sugar. This will give it a golden brown color and a bit of extra sweetness.
Bake the Galette: Bake in the preheated oven for 35-40 minutes, or until the crust is golden brown and the filling is bubbly.
Cool and Serve: Let the Rustic Blueberry Lemon Galette cool on the baking sheet for at least 15 minutes before slicing and serving.