Combine flour, cornmeal, sugar, and salt in a bowl, whisking together to distribute the salt evenly and prevent clumping. Add the chilled butter cubes and use a pastry cutter, two knives, or your fingertips to work the butter into the flour until the mixture resembles coarse breadcrumbs with some pea-sized pieces of butter still visible—this creates those desirable flaky layers in the final crust. Stir in the buttermilk just until shaggy clumps form; don't overmix, as this will toughen the dough. Shape the dough into a flat disc, wrap it in plastic wrap, and refrigerate for at least 1 hour (I often make this dough the night before, which makes rolling it out much easier).

While the dough chills, combine the blueberries, sugar, cornstarch, lemon zest, lemon juice, vanilla extract, and cinnamon in a bowl. Gently toss until the blueberries are evenly coated and the cornstarch dissolves into the juices—this will thicken the filling as it bakes and prevent a soggy galette. I like to let this mixture sit for 10-15 minutes so the flavors meld together, but you can prepare it whenever is convenient for your timeline.

Preheat your oven to 425°F and line a baking sheet with parchment paper. Remove the chilled dough from the refrigerator and place it on the prepared baking sheet. Using a rolling pin, gently roll the dough into a 12-inch circle, working from the center outward and rotating the dough occasionally to maintain an even thickness. Don't worry about perfect symmetry—rustic edges are part of the galette's charm.

Pour the blueberry filling from Step 2 into the center of the dough, leaving a 2-inch border around the edges. Fold the dough edges up and over the filling, working your way around the circle and slightly overlapping the folds as you go to create a rustic, gathered appearance. The filling will peek through the center, which is exactly what you want. Make sure the folded edges seal properly so filling doesn't escape during baking.

Whisk together the egg and milk to create an egg wash, then brush it evenly over all the exposed dough surfaces—this creates a beautiful golden-brown color and professional finish. Sprinkle the coarse sugar over the egg-washed dough, which will add a delightful crunch to each bite. Transfer the baking sheet to the preheated 425°F oven and bake for 28-32 minutes, until the crust is deep golden brown and the filling is bubbling slightly at the edges.

Remove the galette from the oven and let it cool on the baking sheet for about 10 minutes—this allows the filling to set slightly without the crust becoming too hard or difficult to slice. Serve the galette warm or at room temperature, either as-is or with a dollop of whipped cream or vanilla ice cream on the side.