Roll out your pie crust and fit it to the pie plate. Chill for 30 minutes. Line with foil and fill with dried beans or pie weights. Place on a baking sheet and bake for 18 minutes. Remove foil and let cool while you prepare the filling.
In a large mixing bowl, combine sugar and 1 tablespoon lemon zest, rubbing it with your fingers to disperse the lemon oils. Whisk in flour and salt.
Add eggs and whisk until smooth. Pour in buttermilk, melted and cooled butter, and vanilla, and whisk again until completely blended.
Pour filling into crust. If desired, add a pie shield to prevent the edges from over-browning.
Carefully slide the pie, on the baking sheet, into the oven. Bake for 50 minutes, until golden brown. The edges should be set with the center just barely jiggly.
Remove from the oven and let cool completely to set. The pie will deflate slightly as it cools.
Before serving, sprinkle on grated or ground nutmeg. Top with whipped cream, if desired.