Preheat your oven to 350°F (175°C). Place your 9-inch unbaked pie crust in a pie dish if it’s not already in one. Crimp or flute the edges, then chill the crust in the refrigerator while you mix the filling to help keep the crust flaky.
1 9-inch unbaked pie crust
In a medium bowl, whisk together the granulated sugar, all-purpose flour, and fine sea salt until well combined and no lumps of flour remain.
1 1/2 cups granulated sugar, 3 tablespoons all-purpose flour, 1/4 teaspoon fine sea salt
Add the eggs to the sugar mixture and whisk until the mixture looks thick, pale, and well blended. A hand whisk is sufficient; no mixer is needed.
3 large eggs
Pour in the buttermilk and vanilla extract, then add the lemon zest and nutmeg or cinnamon if using. Whisk until everything is fully combined and the mixture resembles a thin custard base.
1 cup whole buttermilk, 1 1/2 teaspoons pure vanilla extract, 1/2 teaspoon lemon zest, 1/4 teaspoon ground nutmeg
Slowly stream in the melted, slightly cooled butter while whisking constantly. Continue whisking until the filling looks glossy and smooth.
1/2 cup unsalted butter
Place the chilled pie crust on a baking sheet to make it easier to move and to catch any spills. Pour the buttermilk custard filling into the crust; it will be quite full, so handle carefully.
1 9-inch unbaked pie crust, 1 1/2 cups granulated sugar, 3 tablespoons all-purpose flour, 1/4 teaspoon fine sea salt, 3 large eggs, 1 cup whole buttermilk, 1/2 cup unsalted butter, 1 1/2 teaspoons pure vanilla extract, 1/2 teaspoon lemon zest, 1/4 teaspoon ground nutmeg
Bake at 350°F (175°C) for 45–55 minutes, or until the edges are set and the center still has a gentle jiggle when you lightly shake the pan. If the crust edges brown too quickly, tent them with foil or use a pie shield during the last 15–20 minutes of baking.
Transfer the baked pie to a wire rack and let it cool completely for at least 2 hours. The custard will continue to set as it cools; slicing too early can result in runny pieces.
For the cleanest slices and creamiest texture, refrigerate the cooled pie for an additional 2–4 hours or overnight. Serve at room temperature or slightly chilled, topped with whipped cream, berries, powdered sugar, or a drizzle of caramel or honey if desired.
Lightly sweetened whipped cream, Fresh berries or berry sauce, Powdered sugar, Caramel or honey