Combine flour, cornmeal, and salt in a bowl with a pastry blender. Add cold butter and combine until coarse crumbs form.

Drizzle in cold water, a little at a time, stirring to combine with a fork, until dough comes together.

Wrap dough into a disk, chill in refrigerator for 30 minutes.

Preheat the oven to 400°F (200°C). Prepare parchment-lined baking sheet.

Mix blueberries, sugar, lemon juice, lemon zest, and cornstarch in a bowl.

Roll dough on floured surface into a 15-inch circle.

Place blueberry mixture at center of dough, folding and pleating edges around the filling.

Brush dough with egg-water mixture and sprinkle with demerara sugar.

Bake in oven until golden (about 45 minutes).