I used to receive a lot of promotional recipe card sets in the mail. This recipe comes from one of those, Great American Home Baking. The orange adds a little twist to the normal lemon meringue pie, and the zest adds some bite. Very good!
Preheat oven to 325*F. Prick the piecrust all over with a fork. Bake until just beginning to brown, 8 - 10 minutes.
Meanwhile, place the egg yolks in a medium bowl. Whisk until well beaten. Gradually whisk in the sweetened condensed milk. Whisk in the orange juice concentrate and lemon juice. Stir in the orange and lemon zest, if desired. Pour the filling into the partially baked crust. Bake until partially set, 10 - 15 minutes. Remove from the oven. Increase oven to 350*F.
Place the egg whites and cream of tartar in a large bowl. Beat with an electric mixer set on high speed until soft peaks form, 2 - 3 minutes. Reduce speed to medium. Gradually beat in the sugar, one tablespoon at a time, until stiff, glossy peaks form and the sugar is dissolved, about 4 minutes longer.
Immediately spread the egg white mixture over the hot pie filling, carefully sealing the meringue to the edge of the crust. Bake until the meringue is lightly golden all over, 15 - 20 minutes. Cool for 1 hour. Chill for at least 3 hours before serving.