Make the piecrust: Place all the ingredients except the egg yolk and ice water into a food processor fitted with a metal blade; pulse at 1-second intervals until the mixture resembles coarse crumbs. (Alternately, whisk together flour, sugar, and salt in a medium bowl. Cut in the butter and shortening with a pastry blender or 2 knives used scissors-fashion, until the mixture resembles coarse crumbs. Stir in the yolk and ice water.)
Add the egg yolk, and sprinkle on 1 tablespoon of the water. Pulse just until dough begins to clump together (add a little more water, if necessary). Don't overwork--this will make the dough tough. Turn dough out onto a large piece of plastic wrap, and press into a large, flat, hamburger-shaped disc. Wrap tightly, and refrigerate 30 minutes (or up to 2 days).
At least 20 minutes before baking, position a rack in the middle of the oven. Preheat the oven to 400°. Place the dough on a clean tea towel lightly dusted with flour. Roll out to a 12- to 13-inch round. Transfer to a lightly buttered 9-inch glass pie plate. Gently press the pastry against the sides and bottom of the plate.
Trim away all but 1 inch of overhanging dough. Turn the edge under, and use a fork or your fingers to crimp the edge. Place in freezer 5 minutes. Place an 8-inch square piece of buttered foil, butter side down, on top of the crust bottom. Bake at 400° for 8 minutes; remove from oven. Discard foil, and lightly prick the bottom and sides of the crust with a fork. Bake 5 to 8 minutes longer, until surface is dry but not brown. Let crust cool on a wire rack while you make the filling. Position a rack on the lowest shelf in the oven; reduce oven temperature to 350°.
Whisk together the filling ingredients in a large bowl until smooth, and pour into the piecrust. Carefully return the pie to the oven. Bake at 350° for 55 to 65 minutes, or until the center is firm and not jiggly. Transfer the pie to a wire rack to cool completely. To serve, cut into 8 wedges.