Cream together the sugar and butter. Mix in a separate bowl with a whisk, the zest of 1 lemon, juice of 1 1/2 lemons, 2 egg yolks, milk, salt and flour. Add lemon mixture to sugar and butter. Mix thoroughly. In a separate bowl whip the egg whites until soft peaks form. Fold the egg whites into the lemon/sugar mixture. Pour into unbaked pie shell.
Bake at 425F for 15 minutes, then reduce to 350F for 30 minutes or until pie tests done.