Lemon icebox pie is an old-school Southern favorite, and like many such vintage recipes, it enjoys periodic revivals as diners seek out different kinds of down-home comfort food. Although there is a simplified, uncooked version of the dessert using cream cheese, most use a cooked filling that varies from traditional lemon pie in its use of sweetened condensed milk. If your pie filling doesn't set properly, you can try again using an additional egg yolk or a starch thickener.
Spoon the un-set filling out of your pie shell into a bowl or large measuring cup. If it's on a crumb crust, it might be simpler to pour it through a strainer and strain out the crumbs, then start over with a fresh crust. If your filling is still in the original mixing bowl or double boiler, set it aside near your work space.
Separate an egg, and place the yolk in a clean mixing bowl or double boiler, large enough to hold the amount of filling you've made. Whisk it with 1 tablespoon of water or lemon juice, until light-colored and foamy.
Whisk a few tablespoons of the failed filling into the new egg yolk, ensuring it's well mixed. Pour the warm filling into the egg yolk mixture in a slow, fine stream, stirring constantly. If your filling had cooled, heat it gently until it's warm to the touch before incorporating it into the egg mixture.
Transfer the double boiler or mixing bowl to the top of a saucepan filled with simmering water. Whisk or stir the filling gently until it thickens, at a temperature of about 140 degrees Fahrenheit.
Pour the newly thickened filling into the original crust, or a new crust if necessary. Chill for four to six hours, until completely set, before slicing.
Scoop the filling from your pie shell carefully into a bowl. If the un-set filling is still in your mixing bowl or double boiler, set it to the side of your workspace.
Whisk 1 to 2 tablespoons of cornstarch into 4 tablespoons of cold water, until it forms a smooth slurry with no lumps. Combine the cornstarch mixture with an equal quantity of the failed filling, then whisk this mixture thoroughly into the main bowl of filling.
Place the bowl or double boiler containing your filling over a saucepan filled with simmering water. Stir the filling diligently as it reheats. At a temperature between 140 F and 150 F, the cornstarch will suddenly begin to gel, thickening the filling dramatically. Heat for another two minutes, stirring occasionally, to ensure the cornstarch has gelled completely.
Pour the thickened filling into a prepared pie shell. Refrigerate the pie for four to six hours before slicing.