1. Crust: Preheat oven to 300 degrees with rack in center. Lightly brush a 9-inch pie plate with butter. Whisk together egg whites and 1 tablespoon cold water with a mixer 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.
2. Transfer egg-white mixture to prepared pie plate; spread along bottom and up sides to form crust. (Don’t spread past rim of pan.) Bake meringue until crisp and light golden on outside, about 40 minutes. Turn off heat and let cool in oven 1 hour, then transfer to a wire rack and let cool completely.
3. Filling: Meanwhile, whisk egg yolks in a medium saucepan (off heat) until thickened and pale yellow, 1 to 2 minutes. Whisk in sugar and lemon zest and juice. Place over medium heat and cook, stirring constantly with a wooden spoon, until mixture is very thick, about 10 minutes. Transfer to a large bowl. Cover with plastic wrap, pressing it directly onto surface of curd. Refrigerate until thoroughly chilled, at least 1 hour and up to 1 day.
4. Whisk curd until smooth. Whip cream with a mixer on high speed until soft peaks form, about 30 seconds. Working in batches, gently fold whipped cream into curd. Fill meringue crust with lightened curd; smooth top. Refrigerate, loosely covered, at least 8 hours and up to 1 day.
5. Topping: Whip cream and sugar with a mixer on high speed until stiff peaks form, about 40 seconds. Spread over pie. Finely grate lemon zest over top. Slice with a chef’s knife, wiping blade between cuts, and serve.