For pâte brisée:
1 1/4 cups all-purpose flour
3/4 stick (6 tablespoons) cold unsalted butter, cut into bits
2 tablespoons cold vegetable shortening
1/4 teaspoon salt

raw rice for weighting the shell

For the filling
1 cup sugar
3 tablespoons quick-cooking tapioca
1/2 teaspoon vanilla
5 cups 1/2-inch chunks trimmed fresh rhubarb (about 2 pounds) or thawed frozen rhubarb
2 tablespoons unsalted butter, cut into bits

For the streusel topping
1/4 cup firmly packed light brown sugar
1/2 cup all-purpose flour
2 tablespoons unsalted butter, cut into bits
1/4 teaspoon cinnamon
1/4 cup finely chopped walnuts
an egg wash made by beating 1 large egg with 1 tablespoon water
about 1/4 cup red currant jelly, melted

Roll out the dough 1/8 inch thick on a lightly floured surface, fit it into a 9-inch (1-quart) pie plate, and crimp the edge decoratively. Line the shell with foil, fill the foil with the rice, and bake the shell in the middle of a preheated 425°F. oven for 10 minutes. Remove the rice and foil carefully and bake the shell for 3 to 5 minutes more, or until it is pale golden.

Make the filling:
In a large bowl stir together the sugar, the tapioca, a pinch of salt, the vanilla, the rhubarb, and the butter until the mixture is combined well and let the filling stand, stirring occasionally, for 15 minutes.

Make the streusel topping:
In a large bowl stir together the brown sugar, the flour, the butter, the cinnamon, the walnuts, and a pinch of salt and blend the mixture until it resembles coarse meal.

Spoon the filling into the shell, sprinkle the streusel topping around the outer 2 inches of the filling, leaving the center exposed, and brush the edge of the shell with some of the egg wash. Bake the pie on a baking sheet in the lower third of a preheated 450°F. oven for 15 minutes, reduce the heat to 350°F., and bake the pie for 50 minutes to 1 hour more, or until the filling is bubbling and the rhubarb is tender. Brush the exposed (center) portion of the filling with the jelly and let the pie cool on a rack for 2 hours.

Makes the pâte brisée:
In a large bowl blend the flour, the butter, the vegetable shortening, and the salt until the mixture resembles meal. Add 2 tablespoons ice water, toss the mixture until the water is incorporated, adding more ice water if necessary to form a dough, and form the dough into a ball. Dust the dough with flour and chill it, wrapped in wax paper, for 1 hour.