The peaks of the meringue turn a tantalizing golden brown in the oven — perfect for plucking off the pie by sneaky fingers. Beware of peak thieves!
In a medium bowl, combine the lemon juice and zest; gradually whisk in the sweetened condensed milk.
Separate the eggs, putting the whites into the large bowl of an electric mixer. Add the yolks to the sweetened condensed milk mixture and stir it well. Pour the mixture into the chilled crust.
Add the cream of tartar to the egg whites in the mixer bowl, and beat them on high speed until stiff peaks form. Add the sugar gradually, and beat the meringue until it is stiff and glossy.
With the flat of the knife, pull up little peaks in the meringue. Bake the pie for 15 minutes, until the peaks are light golden brown.
Let the pie cool completely before you serve it.