"This is an old recipe that I've tweaked to be gluten free. I use an oval serving dish that is 8x13 inches at the widest points of the oval so obviously the amount of graham crackers that you use for the base are entirely to your own liking and you can tweak accordingly."
Place graham crackers into a resealable plastic bag and crush into crumbs using a rolling pin. Pour into a bowl and add enough melted butter so crumbs stick together. Press mixture firmly into your serving dish to form the crust. Place in the freezer until firm, about 20 minutes.
Preheat oven to 325 degrees F (165 degrees C). Remove crust from freezer.
Whisk condensed milk, egg yolks, lemon juice, and lemon zest together in a bowl; pour over crust.
Bake in the preheated oven until just set, 10 to 15 minutes. Remove pie from oven. Keep oven on.
Beat egg whites in a glass, metal, or ceramic bowl until medium peaks form. Add sugar gradually, continuing to beat until stiff peaks form. Whisk in salt and cream of tartar gently. Spread meringue over pie. Use a fork to create peaks in the meringue.
Increase oven temperature to 375 degrees F (190 degrees C).
Return pie to the oven and bake until meringue topping is golden brown, about 15 minutes. Turn off oven and leave the pie inside to cool, about 30 minutes.
If you haven't got cream of tartar, you can use a small amount of vinegar or lemon juice.
I've seen several cooking programs, some of which say to leave the door open while cooling, but I've always left the door shut and it seems to work perfectly. Once cooled there will probably be lots of cracks over the meringue but it tastes fantastic!
Feel free to use regular graham crackers if you don't need the pie to be gluten free.