How to Make It
Prepare the Crust: Preheat oven to 350°F. Stir together crushed graham crackers, butter, and sugar in a medium bowl until combined.
Transfer Crust mixture to a 9-inch tart pan or pie plate. (If using a tart pan with removable bottom, surround bottom of pan with aluminum foil to prevent Filling from leaking.) Press mixture evenly in bottom and up sides of pan. Freeze until set, 5 to 10 minutes. Brush bottom and sides of Crust with egg white. Bake in preheated oven 10 minutes. Cool on a rack while preparing Filling.
Prepare the Filling: Whisk together sugar, cornstarch, and salt in a medium saucepan. In a medium bowl, whisk together water, egg yolks, lemon juice, and lime juice. Gradually whisk yolk mixture into sugar mixture until combined. Cook over medium-high, whisking often, until Filling thickens and just begins to bubble, 6 to 8 minutes. Cook, whisking constantly, 1 minute more.
Remove from heat. Whisk in butter, lemon zest, and lime zest. Allow to cool 30 minutes before pouring into Crust.
Prepare the Meringue: Beat together egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form. Gradually add sugar, 1 tablespoon at a time, beating until stiff peaks form.
Spread Meringue over Filling, being sure to spread to outside of Crust to seal in Filling and prevent shrinkage. Bake at 350°F until Meringue is golden brown, 14 to 15 minutes. Cool before serving.