Mix 1/2 cup of the sugar and the flour smoothly together in a medium-size saucepan.
Stir in the lemon juice, milk and lemon rind.
Cook and stir until smoothly thickened.
Remove from heat and stir in the butter.
Cool slightly and add beaten egg yolks.
Beat egg whites until stiff and moist with the remaining sugar.
Fold in the lemon custard and pour into the baked shell.
Bake at 350F for 35 to 40 minutes.
Cool completely and spread with sweetened whipped cream or whipped topping.