In a large bowl, whisk together flour, 1 tbsp granulated sugar, and salt. Add the very cold, cubed butter and cut it into the flour mixture using a pastry blender, two knives, or your fingertips, until it resembles coarse crumbs with some pea-sized pieces of butter remaining. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Don't overmix! Form the dough into a disk, wrap it in plastic, and let it chill in the fridge for at least 15 minutes while you prepare the filling (or up to 30 minutes if you have time).

While the dough chills, preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. In another large bowl, gently combine the fresh blueberries with 1/4 cup granulated sugar, cornstarch, lemon juice, and vanilla extract. Toss gently until the berries are evenly coated.

On a lightly floured surface, roll out the chilled dough into a rough 12 to 14-inch circle, about 1/8-inch thick. Don't worry about it being perfectly round; the rustic look is part of its charm! Carefully transfer the dough to the prepared baking sheet.

Spoon the blueberry filling into the center of the dough, leaving a 2-inch border all around the edge. Gently fold the edges of the dough up and over the filling, pleating as you go, to create a rustic, free-form crust. The center of the filling will remain exposed.

In a small bowl, lightly beat the large egg. Brush the folded dough edges with the egg wash, then sprinkle generously with turbinado sugar for a lovely crunch and sparkle.

Bake for 35-40 minutes, or until the crust is deeply golden brown and the blueberry filling is bubbly and thick. If the crust starts to brown too quickly, you can loosely tent it with foil. Let the galette cool on the baking sheet for at least 15-20 minutes before slicing and serving. This allows the filling to set properly.