Prepare pie crust according to recipe instructions. As per recipe, prepare, chill, roll out, and arrange into a 9 ½" pie plate (do not bake). Return to refrigerator to chill while you prepare apple pie filling.

Preheat oven to 425F (220C)

In a large mixing bowl, whisk together sugars, flour, cinnamon, allspice, and nutmeg.

Add sour cream, lemon juice, and vanilla extract, and stir well until completely combined (mixture will be like a loose paste).

Add apple slices and stir until all apples are coated with mixture.

Pour apple filling into prepared pie crust, mounding the apples in the center of the pie. Set aside while you prepare crumble topping.

Combine flour and brown sugar in the basin of a food processor¹ and pulse to combine.

Sprinkle butter pieces over the mixture. Pulse until coarse crumbs form.

Add walnuts and pulse briefly (don't overdo it or the mixture will lose its crumbliness and clump together)

Take your crumble topping and sprinkle it evenly over the pie filling.  Use your hands to gently pat down the topping.

Bake on 425F (220C) for 15 minutes. Without opening the oven door, reduce heat to 350F (175C) and continue to bake another 40 minutes, or until apples are tender (use a skewer to pierce the pie and check the apples).  Keep an eye on your pie crust as it bakes, the crust is prone to browning faster than the pie is finished, and if the crust is becoming too dark you should remove the pie from the oven and cover the crust with foil before returning to oven to finish baking.

Enjoy warm, preferably topped with vanilla ice cream.