Pour the flour, sugar, and salt into the work bowl of a food processor. Pulse the machine a few times just to combine. Add the cold butter, and then pulse the machine 5 times or so just to break up the butter. With the machine running, add the beaten egg through the feed tube. Stop processing when the sound of the machine changes from a loud roar to a soft roar -- 10-15 seconds.
Pour the crumbly dough onto a non-floured work surface. Knead the dough briefly to incorporate any stray bits of flour. Then form the dough into a ball, flatten it into a disk, and wrap it in cling film. Chill for 30 minutes to 1 hour.
Using a rolling pin, roll the dough into a 12-inch-diameter circle. Then transfer the circle to an un-greased 9-inch-diameter removeable bottom tart pan. Do not stretch the dough -- simply ease it into the pan. Gently press the sides of the dough against the wall of the pan. Run the rolling pin over the pan to remove any excess dough. Prick the bottom (not the sides) of the dough all over with the tines of a fork. Cover with aluminum foil and fill with pie weights or beans. Chill in the refrigerator for 1 hour (or freeze for 30 minutes).
Center the oven rack. Preheat the oven to 425°F.
Put the tart pan on a baking sheet. Bake just until the dough firms up -- 15-20 minutes. Then remove the foil and beans, and continue baking until the dough colors lightly and feels dry to the touch -- 15-20 minutes. Cool completely.