Step 1

Combine flour, cornmeal, and salt in a bowl with a pastry blender. Add cold butter and combine with the pastry blender until coarse crumbs form. Drizzle in cold water, a little at a time, stirring to combine with a fork, until dough comes together.

Step 2

Turn dough onto a work surface and bring together with your hands; press into a disk of dough.

Step 3

Wrap dough in plastic wrap and chill in the refrigerator for about 30 minutes.

Step 4

Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper or a Silpat® baking mat.

Step 5

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

Step 6

Remove dough from the refrigerator and roll on a lightly floured work surface into a 15-inch circle, about 1/8 inch thick.

Step 7

Place blueberry filling into the center of the dough, leaving 3 to 4 inches of space around the border of dough. Fold dough up and around the berries, rotating a few inches and make another fold, overlapping the first; repeat around the dough until pleats are formed.

Step 8

Combine beaten egg and water in a small bowl and brush on the dough. Sprinkle on demerara sugar.

Step 9

Bake in the center of the preheated oven until golden, about 45 minutes.