Combine flour, sugar, and salt in a food process. Pulse briefly to blend ingredients.
Scatter butter pieces evenly over the top of the mixture and pulse until coarse crumbs form.
Add sour cream and pulse again until the mixture begins to clump together.
Remove the dough from the food processor (be careful of the blade!) and use your hands to form it into a flat disc. Wrap in cling wrap and refrigerate for 30 minutes.
Meanwhile, preheat your oven to 375F (190C) and begin preparing your filling and topping.
Prepare your filling by combining diced apples, sugars, flour, cinnamon, allspice and lemon juice in a large bowl. Stir until well-combined and set aside.
Combine the flour, walnuts, brown sugar, and salt in the clean canister of your food processor and briefly pulse to combine.
Sprinkle butter over the mimxture and pulse until ingredients are beginning to cling together. Set aside.
Once your pie dough has chilled, remove from the refrigerator and place on a clean, lightly floured surface. Dust the dough with flour and roll out to be slightly larger than your jelly roll pan (I usually aim for slightly larger than a 13x18" rectangle and trim the edges to make a neat rectangle).
Transfer rolled out dough to your jelly roll pan and make sure pie crust lies flat along the bottom of the pan and goes up the sides of the pan.
Give your apples that have been waiting a good stir and then evenly pour into pie dough-lined pan.
Top with prepared walnut crumble by using your hands to evenly sprinkle over apple filling.
Transfer to oven and bake on 375F (190C) for 50-55 minutes or until top is golden brown and the apples inside are tender when pierced with a fork*. Serve warm, preferably topped with vanilla ice cream!
*check your pie at 45 minutes and if the topping or the crust is beginning to look brown but the apples are still not tender, cover with foil and continue to bake until the apples are tender. This will keep your topping/crust from becoming burnt.