Makes 1 x 9-inch pie.
For crust, combine flour, sugar and salt. Cut in chilled butter until a rough crumbly texture and little bits of butter are still visible. In a small bowl, whisk egg and lemon juice. Pour all at once into flour mixture and combine just until dough comes together. Shape dough into a disc and chill for at least one hour.
Preheat oven to 400 °F. On a lightly floured surface, roll out dough to just less than ¼ inch thick. Sprinkle a 9-inch pie shell with flour and line with dough. Tuck in rough edges and crimp (pinch) with your fingers. Put pie shell in freezer just for 10 minutes to rest and firm up. Once chilled, line pastry with aluminum foil (and have foil hang over crust edges to protect it) and weigh down with pie weights, dried beans or raw rice. Bake for 10 minutes, then reduce oven temperature to 375 °F and bake 15 minutes more. Remove aluminum foil and weights and bake 10 minutes more, to dry out center of the shell. Cool completely before filling.
For filling, whisk sugar, water, and cornstarch in a heavy-bottomed saucepot. Whisk in egg yolks and salt and cook over low heat for 5 minutes, whisking constantly. Increase heat to medium and, still whisking, cook until filling becomes glossy and thick, about 5 more minutes. Remove from heat and strain. Stir in lemon zest, lemon juice and butter until butter dissolves. Pour immediately into cooled pie shell and let cool 15 minutes. Chill completely before finishing with meringue, about 4 hours.
For meringue, preheat oven to 350 °F. Whip egg whites with cream of tartar until foamy. While whipping, gradually pour in sugar and whip on one speed less than highest until whites hold a stiff peak (the meringue stands upright when whisk is lifted). Whisk in cornstarch and dollop over chilled lemon filling. Use sweeping motions with your spatula to create swirls and peaks that look so enticing once browned. Bake pie for 10 minutes, just until meringue browns lightly. Let pie cool or chill until ready to slice.