Preheat oven to 300°F (150°C) with a rack in the center. Lightly brush a 9-inch pie plate with butter. Using a mixer, whisk egg whites and 1 tablespoon cold water on high speed until foamy (about 30 seconds). Add cream of tartar and continue to beat until soft peaks form (about 1 minute). Gradually add sugar and beat until thick, glossy peaks form (about 5 minutes).

Transfer egg-white mixture to the prepared pie plate, spreading it along the bottom and up the sides to form a crust (but not past the rim). Bake until the meringue is crisp and lightly golden on the outside (about 40 minutes). Turn off the oven and let it cool inside for 1 hour, then transfer to a wire rack to cool completely.

In a medium saucepan (off heat), whisk egg yolks until thick and pale yellow (1-2 minutes). Whisk in sugar, lemon zest, and lemon juice. Place over medium heat, stirring constantly with a wooden spoon, until the mixture thickens (about 10 minutes). Transfer to a large bowl and cover with plastic wrap, pressing it directly onto the surface of the curd. Refrigerate until fully chilled (at least 1 hour, or up to 1 day).

Whisk curd until smooth. In a separate bowl, whip heavy cream on high speed until soft peaks form (about 30 seconds). Gently fold the whipped cream into the curd in batches. Fill the meringue crust with the lemon curd mixture, smoothing the top. Cover loosely and refrigerate for at least 8 hours, or up to 1 day.

Whip heavy cream and sugar on high speed until stiff peaks form (about 40 seconds). Spread over the pie and finish with finely grated lemon zest. Slice with a sharp knife (wiping between cuts) and enjoy!