Combine the flour, sugar, and salt for the crust. Work in the cold butter until the mixture resembles coarse crumbs, then add the ice water gradually until the dough holds together when pressed.
Shape the dough into a disk and refrigerate for 30 minutes. Roll it into a 12-inch circle, fit it into a 9-inch pie plate, and chill for another 15 minutes while the oven preheats to 375°F.
Line the crust with parchment paper and pie weights. Bake for 15 minutes, remove the parchment and weights, then bake for another 6 minutes until the base looks dry but is not deeply browned.
Reduce the oven to 350°F. Whisk the granulated sugar, cornstarch, flour, nutmeg, and salt together until evenly combined.
Whisk in the buttermilk, heavy cream, melted butter, lemon juice, and vanilla extract until the filling is completely smooth. Pour it into the warm partially baked crust.
Bake for 42 to 48 minutes, until the outer portion of the filling is set and the center still has a slight wobble. If the crust begins browning too quickly, loosely cover the outer edge with foil.
Cool the pie completely at room temperature, then refrigerate for at least 3 hours before serving so the creamy custard filling can fully set.
