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 the 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 for 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.
Sweetened Whipped Cream
In a chilled bowl combine whipping cream, sugar, and vanilla. Beat with an electric mixer on medium speed until soft peaks form (tips curl).
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.
Pastry for a Single-Crust Pie
In a medium bowl stir together flour and salt. Using pastry blender, cut in shortening and butter until pieces are pea size.
Sprinkle 1 tablespoon of the water over part of the flour mixture; toss with a fork. Push moistened pastry to side of bowl. Repeat moistening flour mixture, using 1 tablespoon of the water at a time, until flour mixture is moistened. Gather flour mixture into a ball, kneading gently until it holds together.
On a lightly floured surface use your hands to slightly flatten pastry. Roll pastry from center to edges into a circle about 12 inches in diameter.
Wrap pastry circle around the 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 even with the plate's edge. Crimp edge as desired. Do not prick pastry. Fill and bake as directed in recipes.