In a 1.5qt saucepan, combine the lemon juice, water, sugar, salt, and lemon zest.
Whisk together egg yolks and cornstarch in a mixing bowl large enough to fit the liquid from step 1.
Bring the lemon juice mixture to a boil over medium heat.
As soon as your lemon juice mixture reaches a boil, slowly pour the mixture into the egg yolks while whisking vigorously.
Once the yolks and liquid are fully incorporated, return the mixture to the saucepan on medium-low heat.
Whisk slowly and continuously, scraping the bottom of the saucepan, until it comes to a boil.
Once the mixture begins to bubble, whisk vigorously for 1 minute before removing from heat.
Whisk the cubed butter into the lemon curd until it is fully melted and incorporated.
Strain the lemon curd through a fine-mesh strainer into a mixing bowl to strain out any chunks.
Pour the lemon curd filling into your fully baked pie crust and refrigerate until set. (~3 hours)