Beat egg yolks; whisk in lemon rind, sweetened condensed milk and lemon juice. Pour into prepared crust. Bake in preheated 325-degree oven for 25 minutes. Remove from oven. Increase oven temperature to 350 degrees. Beat egg whites with cream of tartar until foamy; gradually add sugar, beating until stiff peaks form but are not dry. Spread meringue on top of hot pie, sealing carefully to edge of crust. Return to oven and bake for 10 minutes or until meringue is golden brown. Cool. Chill before serving, if desired.