Heat the oven to 350°F and fit the pie crust into a 9-inch plate, crimping the edge high.
Whisk the eggs, sugar and flour for 1 minute until smooth.
Whisk in the melted butter gradually, then add the buttermilk, lemon juice, vanilla, nutmeg and salt.
Pour the filling into the unbaked crust and set the pie on a rimmed sheet pan.
Bake for 50 to 55 minutes until the top is golden, the edges are set and the center gives a small wobble rather than a liquid wave.
Cool for at least 3 hours before slicing so the custard sets cleanly.