For the cornmeal crust

1. In a medium bowl, whisk together flour, cornmeal, sugar, and salt. Add the cold, cubed butter to the mixture. Using a pastry cutter, cut the butter into the flour mixture until the butter is pea-sized pieces. Pour in the cold buttermilk. Using a fork, bring the dough together just until combined.
2. Separate dough into six equal-sized pieces, form discs, and wrap in plastic wrap. Refrigerate dough at least 1 hour before rolling or up to 3 days.

For the filling

1. Place a rack in the center of the oven and preheat oven to 425°F. Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
2. In a medium bowl, gently toss together blueberries, sugar, cornstarch, lemon zest, and lemon juice. Allow the filling to sit while you roll out your dough.

Assemble the galettes

1. On a lightly floured surface, roll out each dough crust to an 1/8-inch thick circle (or round-ish shape).
2. Using a slotted spoon, pile a heaping 1/3 cup of blueberry mixture onto the center of each crust, leaving a 1.5-inch border. Gently fold the border over the filling, overlapping dough as necessary. Transfer each galette to the prepared baking sheet. Brush edges of dough with egg wash and sprinkle crust and with coarse sugar.
3. Bake for 15 minutes at 425°F, then reduce heat to 375°F and continue baking for an additional 15 minutes. The filling should be bubbly and the crust golden brown.
4. Allow your galettes to cool slightly before serving.
5. Place a scoop of vanilla ice cream on each galette just before serving (optional).