Make the crust: 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. Add 1 more Tablespoon of buttermilk if the dough seems dry. Gently knead the dough a few times on a lightly floured work surface until it all comes together. Shape the dough into a ball and flatten it into a thick disk. Wrap it in plastic wrap and refrigerate it for at least 1 hour and up to 3 days (or freeze up to 3 months).
As the dough chills, prepare the filling: Mix the berries, sugar, cornstarch, and lemon juice together in a large bowl. Cover tightly and let sit until the dough is ready. I usually cover it and keep in the refrigerator during this time.
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 dough into a 12-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.
Spoon the berries (not the juices) into the center of the dough, leaving a 2-3 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 25-28 minutes. Allow to cool on the baking sheet for 10 minutes before slicing and serving.
Make ahead tip: Both the dough and filling can be made ahead of time and chilled in the refrigerator for up to 3 days. The dough can be frozen for up to 3 months after prepared in step 1. Allow to thaw overnight in the refrigerator before rolling out and filling.
Make sure the butter and buttermilk are SUPER cold. Cold ingredients, as well as chilling, help the crust keep its shape.