Heat the oven to 350 degrees F. In a mixer, combine the butter and sugar until the sugar is completely incorporated. Add the egg yolks and mix well to combine. Add the flour
, lemon juice, nutmeg, and salt. With the mixer running, slowly add the buttermilk. In another bowl
, whip the egg whites until they form soft peaks. Pour a little of the buttermilk mixture into the whites; fold gently to combine. Gently fold the egg white mixture into the remaining buttermilk mixture until just combined. Pour the custard into the baked pie shell. Bake in the middle of the oven until the filling is lightly browned and barely moves when the pie is jiggled, 45 to 50 minutes. Cool on a rack and serve warm or at room temperature. Refrigerate leftovers.