Let the buttermilk and eggs come to room temperature 1 hour before making the filling.
Preheat the oven to 350 degrees F. Ease the dough into a 9-inch deep-dish pie plate and crimp as desired. Pierce all over with a fork. Line the dough with foil, then fill with pie weights or dried beans. Bake until the edges are golden, about 20 minutes. Remove the parchment and weights and continue baking until lightly golden all over, about 10 more minutes. Let cool completely on a rack.
Make the filling: Beat the butter and sugar in a stand mixer fitted with the whisk attachment (or use a hand mixer) on medium-high speed until combined, about 1 minute. Add the egg yolks and beat until pale and thick, about 1 minute. Reduce the mixer speed to medium low; add the flour, lemon juice, lemon zest, nutmeg and salt and beat until just incorporated. Increase the speed to medium high and slowly pour in the buttermilk. Scrape down the sides of the bowl and continue beating until combined. Transfer to a large bowl.
Clean the mixer and whisk attachment. Add the egg whites and beat on medium-high speed until stiff peaks form, about 4 minutes. Fold the egg whites into the yolk mixture in three additions until no white streaks remain. Pour the filling into the prepared crust. Transfer to a baking sheet and bake until golden and set around the edges (the center will still jiggle), 45 to 50 minutes, rotating the pie halfway through. Let cool on a rack. Dust with confectioners' sugar before serving.