In a food processor, combine the flour, sugar and salt and pulse several times. Add the vegetable shortening and pulse 5 or 6 times, until the shortening is the size of peas. Add the butter and pulse 5 or 6 times, until the butter is the size of peas. Pour the ice water evenly over the top. Replace the lid and pulse 5 or 6 times, just until moistened.
Transfer the pastry to a lightly floured work surface and knead several times, just until it comes together. Divide the pastry in half and pat it into two 6-inch disks; wrap the disks in plastic and refrigerate until firm, at least 30 minutes, or overnight, or freeze.
Work with 1 disk of pastry at a time. On a lightly floured surface, gently tap the pastry with a rolling pin to flatten it slightly. Dust lightly with flour and roll out the pastry to a 13 1/2-inch round 1/8 inch thick. To line a pie plate, roll the pastry around the rolling pin, then unroll it over a 9- or 10-inch glass pie plate.
If making a double-crust pie, refrigerate the bottom crust while you prepare the top. Roll out the second piece of pastry to a 13 1/2-inch round 1/8 inch thick. Transfer to a wax paper–lined baking sheet. Using a 1/2-inch cutter, stamp out a vent hole in the center.