Preheat oven to 350°F. Roll out pie crust on a floured surface to a 12 inch circle. Place in a 9-inch pie dish, crimping the edges. Place in the freezer while preparing the rest of the pie.
In a large skillet (with a lid), add the apples, lemon juice, sugar, cornstarch, cinnamon, and salt. Cover and cook over medium heat for 6-8 minutes, stirring every couple of minutes or until desired tenderness is reached. The apples will not cook very much further in the oven (see note). Remove from heat and stir in the vanilla. Cool in the refrigerator until cold.
Make the crumble: In a medium bowl, combine all of the crumble ingredients until well combined. I like big chunks of crumble so I like to kind of squeeze little chunks together. Place in the fridge until ready to assemble.
Assemble the pie: Remove pie crust from the fridge and fill with the cooled apples. Sprinkle the crumble on top. Bake on a cookie sheet for 40-45 minutes or until topping starts to turn brown. If you want the crust to brown more quickly you can brush the crust with some milk. If it's getting too brown you can cover loosely with aluminum foil. Let pie cool completely! It will be a runny mess if you don't. You can store the pie at room temperature for one day then it should be moved to the fridge. This can be made one day ahead of time for optimal results. Serve with vanilla ice cream. If you like the pie warm you can heat for 5-10 minutes but it may lose its shape.
Notes
Par-cooking apples releases pectin which allows the apples to hold their shape as they continue to tenderize in baking. Granny Smith apples have a high amount of pectin which is why I like using them.Make ahead: This pie is great made the day before and can sit at room temperature for a day.