Add sugar, butter, and flour and beat until smooth.
Stir in buttermilk, lemon juice, vanilla, and nutmeg. Pour into pie shell.
Bake until center is firm, 40 to 60 minutes. It will read 175° to 180° on an instant read thermometer when inserted just off center.
Remove from the oven and cool for 1 hour before serving.
Notes
Storage:Buttermilk pie should be refrigerated at all times to prevent spoilage. Cover any leftovers carefully with plastic wrap and it'll last for about three days.To Freeze:Store leftover pie in an airtight container or wrap the whole pie plate tightly in one layer of plastic, then one of foil.Freeze for two to three months. Let the frozen pie thaw in the refrigerator overnight.Tips:
Be sure to use real buttermilk.
Be sure to mix the flour and sugar into the eggs before adding the acidic buttermilk and lemon juice. Otherwise, your eggs could curdle and split the mixture.
The best way to check for doneness is similar to checking a cake. Insert a toothpick near the center of the pie—it should come out mostly clean.
Or, use an instant read thermometer - it will be about 180F near the center when done.
You should also test for doneness by doing the "wiggle test." Gently but firmly give the pie pan a nudge. The center should gently wobble, but it shouldn't ripple or move near the edges.