Preheat oven to 400º. Line a baking sheet with parchment paper.

In a large bowl combine blackberries, sugar, lemon zest, lemon juice, cinnamon and cornstarch. Stir until combined; then set aside.

On a lightly floured work surface, roll out pie dough into a large circle about 1/4" thick. Transfer the pie dough to the baking sheet. Spoon the blackberry mixture onto the center of the dough. Gently fold the dough up and around the blackberries, leaving some of the fruit in the center exposed. In a small bowl, whisk together the egg and water. Brush on the pie crust; then sprinkle generously with the sanding sugar. Bake the crostata for about 40 to 50 minutes or until the fruit is bubbly and the crust is golden. Allow the crostata to cool completely before cutting and serving.