1. In the bowl of a food processor, combine the flour, sugar, and salt and pulse once or twice to mix. Add the cold cubed butter to the processor and pulse three or four times, until the butter is in pea-size pieces. Drizzle the water over the flour mixture and pulse a few times, just until the dough comes together in a shaggy ball.
2. Divide the dough into two and wrap each half tightly in plastic wrap, using the plastic wrap to help gather up any loose crumbles of dough. Press each ball into a disk, transfer to the refrigerator, and refrigerate for 30 minutes before rolling (if you refrigerate it longer, let the dough stand at room temperature for 10 minutes before rolling). The doughwill keep, refrigerated, for up to 3 days or it can be tightly wrapped and frozen for up to 3 months. Thaw overnight in the refrigerator before using.
3. On a lightly floured work surface, with a lightly floured rolling pin, roll the dough into a thin, even circle about 12 inches in diameter, beginning in the center and working outward.
4. Preheat the oven to 350°F.
5. In a large bowl, whisk together the sugar, melted butter, eggs, flour, lemon juice and zest, vanilla, salt, cinnamon, and nutmeg. Whisk in the buttermilk, then pour the filling into the unbaked pie crust and bake until the pie is golden brown on top and the center jiggles slightly, 45 to 50 minutes.
6. Transfer to a wire rack and let cool completely before slicing.