Whisk together the lemon juice, lemon zest, sugar, and egg yolks in a medium bowl.
Mix the cornstarch with water until the cornstarch is dissolved.
Place your cast iron skillet on medium heat.
Add the sugar and salt to the skillet. Stir to combine.
Gradually stir the water/cornstarch mixture into the skillet with the sugar and salt, and stir well, to remove lumps.
Stir in the lemon and egg yolks mixture. Whisk continuously to blend all the ingredients well.
Continue to cook over medium heat, stirring constantly to prevent the mixture from scorching or sticking to the skillet. Cook until the filling thickens and becomes smooth.
Once the filling has thickened, remove the skillet from the heat.
Pour the lemon filling into your pre-baked pie shell or graham cracker crust. Smooth it out evenly.
Finish by topping the pie with meringue or whipped cream, as per your preference.
If using meringue, you can bake the pie at 325° until golden brown for about 15-20 minutes.
