MIX FILLING BASE: In a medium saucepan, whisk together 1 cup granulated sugar and 1/4 cup cornstarch. Gradually whisk in 1 1/2 cups cold water until smooth.
ADD YOLKS: Whisk 3 large egg yolks lightly, then add them to the saucepan. Stir until fully combined. Set aside.
PREP LEMON: Measure 1/4 cup lemon juice and 1 teaspoon lemon zest and set aside.
START MERINGUE: In a large, clean mixing bowl or stand mixer fitted with the whisk attachment, beat 4 large egg whites, room temperature until foamy. Add 1/4 teaspoon cream of tartar and continue beating on high speed.
ADD SUGAR TO MERINGUE: With the mixer running, gradually add the 1/2 cup granulated sugar. Beat until stiff, glossy peaks form.
COOK FILLING: While the meringue whips, place the saucepan over medium heat. Whisk constantly until the mixture comes to a low boil. Continue whisking and boil gently for 1 minute.
FINISH FILLING: Remove from heat and whisk in the lemon juice, lemon zest, and 1 tablespoon salted butteruntil smooth.
ASSEMBLE PIE: Immediately pour the hot filling into 1 9-inch pre-baked pie crust. Spoon the meringue over the hot filling, spreading it all the way to the crust edges to seal. Create swirls and peaks with a spoon or spatula.
BAKE: Bake for about 15 minutes, until the meringue is lightly browned.
COOL: Remove from the oven and allow the pie to cool completely before slicing and serving.
Notes
Crust Tip: The pie crust must be fully baked before adding the filling. Follow blind-baking instructions on the crust packaging or recipe.Meringue Tip: Always spread meringue onto hot filling and seal it to the crust edge to help prevent weeping or shrinking.Egg White Tip: Room-temperature egg whites whip best. Make sure the bowl and whisk are completely clean and free of any yolk.Cooling: Allow the pie to cool completely before slicing so the filling sets properly.Storage: Cover loosely with foil and refrigerate for up to 2 days.