How to Make It
Make the crust: Combine the flour, salt, and sugar in a food processor. Add the butter and shortening and pulse to form a coarse crumble. While the motor is running, add the ice water, 1 tablespoon at a time, until the dough just comes together. Turn out onto a lightly floured surface and knead just until smooth. Shape into a disk and wrap in plastic. Refrigerate for 45 minutes.
On a lightly floured surface, roll the dough into a circle about 14 inches in diameter and place in a 9-inch pie plate. Using your fingers, fold over any overhang and crimp the edge. Refrigerate until ready to use.
Make the filling: Heat oven to 375° F.
Peel, core, and slice all the apples 1/4 inch thick. Gently toss them in a bowl with the lemon juice, sugar, and cinnamon. Spoon into the crust and sprinkle the walnuts over the top.
Make the topping: Combine the flour, sugar, cinnamon, and salt in a food processor. Add the butter and pulse to form a medium crumble (with pieces about the size of peas). Sprinkle the topping over the apples and walnuts. Bake for 1 hour, or until the crust is golden brown and the apples are tender.
Make-Ahead Note: The pie can be made up to 1 day ahead. Let cool, then cover and refrigerate. To warm, bring the pie to room temperature, then heat at 350° F for 20 minutes.