With an electric mixer fitted with a whisk attachment, beat the egg whites and cream of tartar together on medium speed for 1 minute.
Increase to high speed until soft peaks form, about 4 minutes.
Add the sugar and salt then continue beating on high-speed until glossy stiff peaks form, about 2 more minutes.
Spread meringue on top of filling.
Make sure you spread the meringue all the way to the edges so that it touches the crust.
Bake for 25-30 minutes or until the meringue is browned on top.
Remove from oven, place on wire rack and allow to cool at room temperature for 1 hour before placing in the refrigerator to chill.
Chill for at least 4 hours before slicing and serving.
Chef's notes: It is very important to make sure that not even a speck of egg yolks get in the whites when separating. Make sure the bowl is free of any oil residue. Even a small amount of film on the bowl could prevent the egg whites from foaming. Meringue doesn't keep well, no matter how hard you try to prevent it, and the meringue will wilt and separate over time. Best to enjoy right away.