How to Make It
Prepare crust: Whisk together flour, dry milk, sugar, and salt in a bowl. Add shortening and butter; use your fingers to combine until well distributed and pea-size clumps form. Stir in 2 tablespoons ice-cold water. Add up to 2 more tablespoons water, as needed, until dough comes together. Knead 2 times on a floured surface. Shape dough into a disk; wrap in plastic wrap. Chill 30 minutes.
Preheat oven to 425°F. Unwrap dough; roll into an 11-inch circle on a floured surface. Place inside a 9-inch glass pie plate. Crimp edges of dough; prick all over with a fork. Bake at 425°F until lightly browned, 10 to 12 minutes. Let cool while preparing filling. Reduce oven temperature to 375°F.
Prepare filling: Whisk together eggs and egg white in a bowl. Whisk together 1 1/2 cups water, sugar, cornstarch, and salt in a saucepan. Bring to a boil, stirring often. Boil until mixture thickens, 1 minute.
Remove from heat; slowly whisk mixture into eggs. Return to pan; cook over medium-low, stirring constantly, 1 minute. Remove from heat; stir in zest, juice, and butter until smooth. Pour into crust.
Prepare meringue: Beat egg whites and cream of tartar with a mixer on medium- high speed until soft peaks form, about 2 minutes. Add sugar, increase speed to high, and beat until stiff peaks form, 1 minute.
Spread meringue over hot filling to edge of crust. Bake at 375°F until top is golden, about 10 minutes. Cool 2 hours.