To prepare the crust, mix the flour, cornmeal, sugar, and salt in a food processor or mixing bowl until combined.

Add the lemon zest and cubed butter, pulsing a few times until the butter is broken down into small, pea-sized pieces.

Lightly beat the egg and add it along with 2 tablespoons of milk to the flour mixture. Pulse until the dough starts to come together, adding a bit more milk if needed.

Transfer the dough onto a large piece of plastic wrap, shape it into a disc, wrap it up, and refrigerate for at least 30 minutes.

For the blueberry filling, combine the blueberries, sugar, lemon juice, lemon zest, and cornstarch in a small bowl, tossing gently to mix.

Preheat your oven to 375º F (190º C).

On a lightly floured piece of parchment paper, roll out the chilled dough into a circle about 12 to 13 inches (30-33 cm) in diameter, turning it often to prevent sticking.

Spoon the blueberry mixture into the center of the dough, spreading it evenly while leaving a 2-inch border around the edge.

Fold the edges of the crust over the filling, creating pleats as you go. If the dough tears, simply pinch it to seal.

The galette should be approximately 10 inches in diameter. Brush the edges with milk or an egg wash, and sprinkle with crystallized sugar if desired.

Carefully transfer the galette onto a large baking sheet and bake for about 45 minutes, or until the edges are golden brown.

Allow the crostata to cool for a few minutes before slicing. It can be served warm or at room temperature, and pairs wonderfully with ice cream or whipped cream.