In a mixer with a whisk attachment, cream the butter well then add the sugar and continue to cream until light and fluffy. Add the yolks and blend in; then mix in the flour. With the mixer on add the buttermilk slowly then the lemon juice and rind.
In a separate bowl, whip the egg whites to stiff peaks. Fold 1/3 of them into the buttermilk batter, then a second and third addition folding after each one.
Pour the filling into the blind baked crust and sprinkle the top with nutmeg. Bake for 40 minutes until the top is golden brown and the filling is firm looking. Cool and serve at room temperature.