• Step 1: Prepare the lemon filling. In a medium saucepan, whisk together the 1 cup of sugar, cornstarch, and salt. Gradually whisk in the water and egg yolks until smooth. Cook over medium heat, stirring constantly, until the mixture thickens and bubbles, about 8-10 minutes.
• Step 2: Remove the saucepan from the heat and stir in the lemon juice and butter until the butter is melted and the mixture is well combined. Pour the hot lemon filling into the pre-baked pie crust, spreading it evenly.
• Step 3: Make the meringue topping. In a large bowl, beat the egg whites with an electric mixer until soft peaks form. Gradually add the 1/2 cup of sugar, beating until stiff, glossy peaks form, about 5-7 minutes.
• Step 4: Spread the meringue over the hot lemon filling, making sure to seal the edges to the crust. Use the back of a spoon to create swirls and peaks in the meringue.
• Step 5: Bake the pie at 350°F for 12-15 minutes, or until the meringue is golden brown. Allow the pie to cool completely before serving, about 2-3 hours.