In a large bowl, whisk together the flour and salt. Add the cold, cubed butter and use a pastry blender (or your fingertips) to cut the butter into the flour until the mixture resembles coarse crumbs.

Slowly add the ice water, one tablespoon at a time, mixing gently after each addition. Stop adding water when the dough just comes together.

Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

In a large bowl, combine the blackberries, granulated sugar, cornstarch, lemon juice, and lemon zest. Gently toss everything together.

Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough into a 12-inch circle.

Carefully transfer the rolled-out dough to a baking sheet lined with parchment paper. Pour the blackberry filling into the center, leaving a 2-inch border. Dot with butter pieces.

Gently fold the edges of the dough over the filling, pleating as you go.

Brush the folded edges with beaten egg wash, then sprinkle with turbinado sugar.

Bake for 40-45 minutes, or until the crust is golden brown and filling is bubbly.

Let the galette cool slightly before transferring to a wire rack to cool completely.