In a large mixing bowl, whisk together all-purpose flour, fine cornmeal, granulated sugar, and salt until well combined. Cut in cold unsalted butter until the mixture resembles coarse crumbs. Stir in buttermilk until the dough starts to clump together.

Shape the dough into a disc approximately 1-inch thick. Wrap it tightly in plastic wrap and refrigerate for at least 1 hour.

While the dough chills, wash and dry the blueberries. In a medium bowl, toss the blueberries with granulated sugar, cornstarch, lemon zest, and lemon juice. Let sit for about 10 minutes.

Preheat your oven to 425°F (190°C). Line a baking sheet with parchment paper.

Lightly flour a clean surface and roll out the dough into a rough circle, about 12-14 inches in diameter. Transfer to the prepared baking sheet.

Spoon the blueberry filling into the center of the rolled dough, leaving a 2-inch border around the edges. Fold the edges of the dough over the fruit, creating a rustic border.

Whisk together the egg and milk to create an egg wash. Brush the outer edges of the folded dough with the egg wash. Optionally, sprinkle coarse sugar over the edges.

Bake for 28-32 minutes, or until the crust is golden brown and the filling is bubbly.

Allow the galette to cool on the baking sheet for about 10 minutes. Slice into wedges and serve warm.