Pour the flour, confectioners’ sugar and salt in the bowl of a food processor. Pulse to combine. Add the diced butter, and pulse 10 times or so just to break the butter up. With the machine running, add the beaten egg. Process until the dough just begins to mass on the blade — 15-30 seconds.
Dump the mass onto a clean (not floured) work surface, gather it into a ball, and then knead it briefly to make sure everything is incorporated.
On a lightly-floured surface, roll the dough into a 13-inch (33cm) circle. Transfer the dough to a rimless, parchment-lined baking sheet, and let it chill in the fridge while you prepare the blueberry filling.
Preheat the oven to 425°F (218°C).
Put the blueberries in a medium bowl. Sprinkle the berries with the granulated sugar, cornstarch, and lemon zest. Stir gently with a spatula to coat.
Sprinkle the tapioca granules over the dough, leaving a 1 1/2 inch (4cm) border all around. Tapioca will insure the galette isn’t runny with juice. Mound the berry mixture onto the round of pastry dough, and then spread it out, with a spatula, to within 1 1/2 inches (4cm) 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 beautifully bronzed, about 25 minutes.. Serve warm or at room temperature, with or without whipped cream or vanilla ice cream.
Pour the flour, confectioners’ sugar and salt in the bowl of a food processor. Pulse to combine. Add the diced butter, and pulse 10 times or so just to break the butter up. With the machine running, add the beaten egg. Process until the dough just begins to mass on the blade — 15-30 seconds.
Dump the mass onto your clean (not floured) work surface, gather it into a ball, and then knead it briefly to insure all stray flour is incorporated. Form the dough into a disc and wrap it in plastic. Refrigerate for 2 hours (or up to 3 days). Or, freeze for 30 minutes (or up to 3 months).