For a Double-Crust Pie:
For a conventional pie with a filling that doesn't require precooking, line pie plate with half of the pastry, as directed above, except do not trim pastry or fold under. Omit the prebaking step. Set pie plate aside. Roll out second half of pastry; cover with waxed paper and set aside. To prepare filling, omit the cornstarch and reduce the raspberries to 1 cup. In a large mixing bowl stir together the sugar and, if using fresh rhubarb, 6 tablespoons of all-purpose flour. (If using frozen rhubarb, increase the all-purpose flour to 1/2 cup.) Stir in the rhubarb, raspberries, and apple. Transfer filling to the pastry-lined plate. Trim pastry even with rim of pie plate. Cut slits in top crust; place crust over filling. Fold top crust under bottom crust; flute edge as desired. Cover edges with foil. Bake in a 375 degree F oven for 25 minutes. Remove foil; bake 20 to 25 minutes more or until top is golden and fruit is tender. Cool on a wire rack.
Bake pie up to 8 hours before serving.