Preheat your oven to 350°F (175°C).
Cream the butter and sugar in a mixing bowl until smooth. I usually use a handheld mixer, but a whisk works fine.
Add eggs one at a time, beating well after each one.
Stir in the flour, followed by the buttermilk, lemon juice, and vanilla. Mix until the filling is smooth and creamy.
Sprinkle in the nutmeg and give it one final stir.
Pour the filling into the pie crust, and place the pie on a baking sheet to catch any drips.
Bake for 45–50 minutes, or until the top is golden and the center is set but still has a slight jiggle.
Cool completely before slicing—this helps the custard firm up perfectly.
Tip: If the edges of your crust start browning too fast, loosely cover them with foil midway through baking.