Pre-bake the crust: Bake your pie crust in a 9-inch pie pan until golden. Let it cool completely before filling.
Make the lemon filling base: Whisk together sugar, cornstarch, and salt in a saucepan. Gradually whisk in water and cook over medium heat until thick and bubbly.
Add lemon juice and zest: Stir in the lemon juice and zest until well combined.
Temper the egg yolks: Whisk a small amount of the hot mixture into beaten egg yolks, then return everything to the saucepan and cook for 2 more minutes.
Finish filling: Stir in butter until melted, then pour filling into cooled crust.
Make the meringue: Beat egg whites until soft peaks form, then gradually add sugar until stiff, glossy peaks hold.
Assemble and bake: Spread meringue over hot filling, sealing to crust edges. Bake at 350°F until peaks are golden, 12–15 minutes.
Notes
For best meringue stability, make sure your mixing bowl is completely grease-free before beating egg whites.