For the crust: Put the flour, granulated sugar and salt in a food processor fitted with a steel blade; pulse to combine. Add the butter and pulse until it is in small pieces and distributed throughout the flour. With the machine running, add the ice water 1 tablespoon at a time until the dough comes together in a ball. Turn out onto a floured surface and gently form into a disc. Wrap in plastic wrap and put in the freezer for at least 1 hour.
Roll the dough out on a lightly floured surface into a 10-inch circle and transfer to a rimmed baking sheet, then refrigerate 20 minutes.
For the filling: Meanwhile, combine the blueberries, granulated sugar, cornstarch, lemon juice and zest in a medium bowl; mix gently until the berries are evenly coated and glossy.
To finish: Preheat the oven to 425 degrees F. Pour the filling into the center of the dough, leaving a 2- to 3-inch border. Gently fold in the edges of the crust all along the circle. Brush the crust with egg and sprinkle with sparkling sugar, if using. Bake until the crust is golden and the filling just begins to bubble, 20 to 22 minutes. Let cool 20 minutes and serve warm or at room temperature.