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.