To get a sweet-and-sour blend that can’t be beat, this slab pie needs to ripen in the fridge. To get the proper texture of firm, somewhat chewy pudding, make sure you get the right set in the oven: a little wiggling at the center, not liquid, but not fully firm. When you tap the corner of the pan (with an oven-mitted hand!), you should see some movement at the pie’s center, nothing dramatic, no waves or even ripples, but just some definite jiggle.
Single-crust version of Shortening-Butter Crust
2 cups granulated white sugar
6 large eggs, at room temperature
3 cups regular cultured buttermilk
16 tablespoons (2 sticks) unsalted butter, melted and cooled to room temperature
4½ teaspoons finely grated lemon zest plus 3 tablespoons fresh lemon juice
2 tablespoons cornstarch
1 teaspoon lemon extract, optional
¼ teaspoon salt
½ teaspoon freshly grated nutmeg
1. Position the rack in the center of the oven. Heat the oven to 350°F.
2. Prepare and shape the crust as directed in a 13 × 18-inch lipped sheet pan. Prick the crust repeatedly with a flatware fork across the flat surface and up the sides. Cover loosely with aluminum foil and bake for 20 minutes. Uncover and set on a wire rack while you prepare the filling. Maintain the oven’s temperature.
3. Using a handheld electric mixer or a stand mixer with the whisk attachment, beat the sugar and eggs in a large bowl at medium speed, until thick and rich, about 6 minutes. Beat in the buttermilk, melted butter, lemon zest and juice, cornstarch, lemon extract (if using), and salt until smooth. Pour into the prebaked crust. Dust the top with the grated nutmeg.
4. Bake until lightly browned and the pie has a slight jiggle at the center, about 30 minutes. Cool in the pan on a wire rack to room temperature, about 1 hour. Cover loosely with plastic wrap and store in the refrigerator for at least 6 hours or up to 2 days before serving.