In a 2- to 3-quart pan, mix 1 1/4 cups sugar with 6 tablespoons cornstarch and salt. Whisk in 1/2 cup cold water, lemon peel, and lemon juice, blending thoroughly. Whisk in the egg yolks. Add the butter and whisk in 1 1/2 cups boiling water.
Set pan over medium-high heat and stir with a flexible spatula until mixture boils, about 7 minutes. Reduce heat to medium and stir filling 1 minute more.
Pour filling into pie crust.
In a small bowl, mix the remaining 1/2 cup sugar with the remaining 2 teaspoons cornstarch.
In a deep bowl, beat egg whites and cream of tartar with a mixer on high speed (use whisk attachment if available) until very foamy and no liquid whites remain in bottom of bowl. Continue to beat at high speed and gradually add the sugar-starch mixture, 1 tablespoon at a time. Then beat until whites hold stiff, glossy peaks.
Spoon meringue onto hot pie filling. With a spatula, swirl meringue to cover filling completely, touching the rim of the pie shell.
Bake in a 325° oven until meringue is browned, about 25 minutes. Set pie on rack until cool, about 3 hours. Serve, or invert a large bowl over pie (take care not to touch meringue) and chill up to 1 day.