Pour the flour, confectioners sugar and salt in the bowl of a food processor. Pulse to combine. Add the diced butter. With the machine running, add the beaten egg.
Roll the dough into a 13-inch circle. Transfer to a parchment-lined baking sheet, and let it chill in the fridge while you prepare the blueberry filling.
Preheat the oven to 425°F.
Put the blueberries in a medium bowl. Sprinkle the berries with the granulated sugar, cornstarch, and lemon zest. Stir gently.
Sprinkle the tapioca granules over the dough, leaving a 1 1/2 inch border all around. Mound the berry mixture onto the round of pastry dough, and then spread it out, with a spatula, to within 1 1/2 inches of the edge. Fold the edge over the filling, making pleats as you go. Brush the edge with the beaten egg, and give it a sprinkling of Demerara sugar.
Bake until the crust is bronzed, about 25 minutes. Serve warm or at room temperature, with vanilla ice cream.
