Prepare the crust as directed and chill while assembling the pie ingredients. The pie crust can be made a day in advance and refrigerated overnight.
For the crumble topping: into the food processor bowl, add the brown sugar, flour, cold butter cubes, and salt. Pulse several times until the butter is about pea-sized. If using walnut halves, add them and pulse a couple times to chop. If using chopped walnuts, add to the bowl as is, no additional chopping. Set aside the topping in the fridge to chill.
For the pie filling: peel, core, and slice the apples, adding them to a large mixing bowl. Add the brown sugar, cinnamon, flour, and lemon juice. Toss to coat and set aside for about 30 minutes.
Begin preheating the oven to 350° F (176° C). Roll out the pie dough and place it in the pie dish. With a slotted spoon, add the apples to the pie plate. Drizzle some of the remaining juices over the apples if there is any, discard the excess. Alternately, cook down the juices in a sauce pan to reduce and drizzle atop the apples.
Pile on the crumble topping and place the pie in the oven. Cover the top loosely with parchment paper for 20 minutes to prevent over browning then remove the paper. Total baking time is approximately 45-50 minutes.