In a stand mixer or mixing bowl, combine sugar, butter, flour and beat until smooth and creamy.
Add the eggs one at a time and mix on low until each egg is incorporated. With the stand mixer on low, stir in the buttermilk, vanilla, salt, lemon zest (optional), and nutmeg (optional).
Bake
Pour the mixture into an unbaked pie shell and place in a preheated oven at 400°F/205°C for about 40 minutes. Adjust baking time as necessary for your range.
Once the pie has set, remove it from the oven cool it on a wire cooling rack. Allow the pie to cool to room temperature before slicing. Then, the pie is ready to serve warm, or you can refrigerate for later. Old Fashioned Buttermilk Pie keeps refrigerated for 4-5 days.