Whisk the flour, cornmeal, sugar, and salt together in a medium bowl. Using a pastry cutter or a couple forks, cut in the butter until the mixture resembles coarse, pea-sized crumbs. Add the buttermilk and stir until the flour is moistened and the dough clumps up. Mix in 1 more Tablespoon of buttermilk if the dough seems dry. Pour crumbly dough onto a lightly floured work surface and bring it together with your hands. Shape the dough into a ball and flatten it into a 1-inch-thick disc. Wrap it in plastic wrap and refrigerate it for at least 1 hour and up to 3 days (or freeze for up to 3 months).

Mix the blueberries, sugar, cornstarch, lemon zest, and lemon juice together in a large bowl. Stir gently until the berries are moistened all over (no powdery lumps of cornstarch or sugar). Place in the refrigerator until ready to use. Tip: Don’t mix the filling too much ahead of time—10 to 15 minutes is fine as you roll out the dough. You do not want the blueberry filling to sit for too long, as the berries will release too much juice.

Preheat oven to 425°F (190°C). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.

On a lightly floured work surface, roll the chilled dough into a 12- to 14-inch circle (or any shape, really!). Trim the rim of the circle to make a clean cut, if desired. Transfer dough to the prepared baking sheet.

Remove the filling from the refrigerator and spoon it into the center of the dough, leaving a 2- to 3-inch border all around. Gently fold the edges of the dough over the fruit, overlapping the dough as necessary. Press gently to seal the edges. Brush the edges with egg wash and sprinkle with coarse sugar.

Bake until the filling is bubbly and the crust is golden brown, about 28 to 32 minutes. Cool on the baking sheet for 10 minutes before slicing and serving. Tastes wonderful served warm with vanilla ice cream or whipped cream.