Preheat the oven to 350°F. Place the pie crust on a baking sheet. Set aside.
In a medium size mixing bowl, sift together the sugar, flour, salt and nutmeg using a whisk.
In a separate mixing bowl, whisk together the buttermilk, butter, eggs, lemon juice and vanilla.
Whisk the wet ingredients into the dry until fully combined. Pour evenly into the pie shell. Sprinkle the top lightly with additional nutmeg, if desired.
Carefully place into the oven and bake for 50-60 minutes OR until the center is set when gently shaken. Check at 30 minute and cover the edges to prevent overbrowning.
Remove from the oven to a cooling rack. Cool completely before cutting.
Serve with a dollop of whipped cream, if desired. Store chilled.
Notes
See how to make your own buttermilk in suggested Baking Ingredient substitutions here.