For the crust: Combine flour, cornmeal, sugar, and salt in a food processor or mixer bowl. Mix to combine. Add the lemon zest and butter cut in pieces, and pulse a few times until the butter is reduced to small (pea-size) pieces. Beat the egg lightly and add with 2 tablespoons of milk to the flour mixture. Pulse and mix until the dough comes together. Add a little bit more milk if necessary. Move the mixture to a large piece of plastic wrap. Make it into a disc. Wrap it in plastic and chill in the refrigerator for at least 30 minutes.

For the blueberry filling: In a small mixing bowl add the blueberries, sugar, lemon juice, lemon zest, and cornstarch. Toss to combine.

Preheat the oven to 375º F (190º C).

Roll out the dough on a large piece of parchment paper lightly floured. Sprinkle the top with flour, and turn the dough often to ensure it doesn't stick to the paper. Roll to a 12-inch to 13-inch (30-33 cm) circle. Spoon the blueberry mixture into the center of the dough and distribute evenly to within 2 inches of the edge. Fold the crust edge over the filling making some pleads. If the dough breaks at any point, don't worry, just pinch it to seal. The tart will come out to be about 10 inches in diameter. Brush the edges with milk (or eggwash if you prefer) and sprinkle with crystallized sugar if you'd like.

Slide the galette into a large baking or cookie sheet. Bake at 375º F (190º C) for about 45 or until golden brown at the edges. Let the crostata cool for a few minutes before cutting. Serve at room temperature, or warm with a scoop of ice cream or whipped cream if you like.