In a large bowl, whisk together the melted butter and sugar. Add the eggs and egg yolk, mixing until combined. Stir in the buttermilk, lemon juice, vanilla extract, and nutmeg. Finally, whisk in the flour until smooth.
Pour the filling into the prepared pie crust. Use aluminum foil or a pie shield to protect the edges of the crust.
Bake the pie for 45-50 minutes, until the edges are set but the center is still slightly jiggly.
Allow the pie to cool for at least one hour. For the best results, refrigerate it overnight.