Preheat oven to 450 degrees F. Prepare Pastry for Single-Crust Pie. On a lightly floured surface, use your hands to slightly flatten dough. Roll dough from center to edge into a 12-inch circle. Wrap pastry circle around rolling pin; unroll into a 9-inch pie plate. Ease pastry into pie plate without stretching it. Trim pastry to 1/2 inch beyond edge of pie plate. Fold under extra pastry. Crimp edge as desired. Prick bottom and side of pastry with a fork. Line pastry with a double thickness of foil. Bake for 8 minutes. Remove foil. Bake for 5 to 6 minutes more or until pastry is golden. Cool on a wire rack.
In a large bowl combine gelatin and boiling water. Stir in sugar until dissolved. Stir in lemon peel and lemon juice. Place bowl in a large bowl of ice water, stirring frequently, until mixture is partially set (consistency of unbeaten egg whites). This will take 10 to 15 minutes.
In a medium bowl beat cream with an electric mixer on medium speed until soft peaks form (tips curl). Fold whipped cream and Rosy Rhubarb Sauce into lemon mixture. Cover and chill 30 to 45 minutes or until mixture mounds when spooned.
Transfer lemon mixture to the baked pastry shell. Cover and chill for at least 8 hours or until firm. Top center of pie with sweetened whipped cream and lemon slices, if desired. Makes 10 to 12 servings.
Pastry for Single-Crust Pie
In a medium bowl stir together flour and salt. Using a pastry blender, cut in shortening until pieces are pea size. Sprinkle 1 tablespoon cold water over part of the flour mixture; toss gently with a fork. Push moistened dough to side of bowl. Repeat with additional cold water, 1 tablespoon at a time, until all of the flour mixture is moistened. Shape into a ball.
Rosy Rhubarb Sauce
In a saucepan, mix sugar, water, and orange peel. Bring to boiling. Add rhubarb. Simmer, uncovered, about 8 minutes or until rhubarb is tender and mixture is thickened, stirring as necessary to prevent sticking. Cool. Chill in the refrigerator. Makes 3-1/2 cups.