To make the pie crust, add the flour, sugar, and salt to the food processor. Pulse until combined. Add the cubed butter. Pulse until the butter and flour resemble a sand-like texture. Add 3 tablespoons of water into the food processor. Process until a dough ball forms. If dough seems too dry, you can add one more tablespoon of water. Take dough out and shape into a 6 inch disc. Cover completely with plastic wrap and refrigerate for at least an hour.
After the dough has chilled, roll pie dough out and shape into a 9 inch pie pan (a deep dish pan is best). Place in the fridge while you prepare the topping and filling.
Preheat the oven to 375 degrees F. Move oven rack one level down from the center. Line a baking sheet with foil. Set aside.
To make the crumble, add the flour, oats, brown sugar, cinnamon, and salt into a medium bowl. Stir with a spatula until combined. Add the melted butter and stir until everything is evenly combined and crumbly. Place in the fridge while you prepare the filling.
To make the filling, add the apples, cinnamon, nutmeg, brown sugar, vanilla, apple cider vinegar, and flour in a very large bowl. Stir with a spatula until everything is combined.
Take the pie pan with crust in it out of the refrigerator and place on the lined baking sheet. Pour filling into the pie crust, leaving the extra liquid behind. Make sure to pack down the apples. I had to use my hands to get it all packed in there. It might seem like a lot of apples, but it cooks down. Use your hands and spread the crumble topping all over the top of the apple pie evenly. Pat it down so it stays put.
Bake in the oven for 50-60 minutes, or until topping is golden brown and apples are slightly tender when poked with a knife. They will soften more as the pie cools and rests. Place on a wire rack to cool for at least 4 hours. The longer it cools, the easier it will be to slice and serve. Enjoy!