In a food processor, combine 1 ½ cups of all-purpose flour and ½ teaspoon of kosher salt. Pulse until well blended. Cut in ½ cup of cold unsalted butter until the mixture resembles coarse crumbs. Gradually add 4–6 tablespoons of ice cold water, pulsing until the dough begins to form a ball. Wrap the dough in plastic wrap and chill in the refrigerator for at least 30 minutes.

While the dough chills, adjust your oven shelf to the center position and preheat to 400°F. Line a baking sheet with parchment paper.

In a mixing bowl, gently fold together 2 cups of fresh blackberries, ½ cup of granulated sugar, 2 tablespoons of cornstarch, 2 tablespoons of fresh-squeezed lemon juice, 1 teaspoon of grated lemon zest, and a pinch of salt.

Remove the chilled dough from the refrigerator and place it on a floured surface. Roll it out into a 12-inch circle, about ¼ inch thick. Transfer the rolled dough to the prepared baking sheet.

Spoon the blackberry filling into the center of the dough circle, leaving a generous 2-inch border around the edges. Fold the edges of the dough over the filling, pleating as you go.

Chill the assembled galette in the refrigerator for 10 minutes. Whisk 1 egg to create an egg wash. Brush the crust with egg wash and sprinkle 1 tablespoon of turbinado sugar on top.

Place the galette in the oven and bake at 400°F for 10 minutes, then reduce to 375°F and bake for an additional 20 minutes, until the crust is golden brown.

Allow the galette to cool slightly for about 10-15 minutes before slicing. Serve warm, optionally topped with a scoop of vanilla ice cream.