Combine flour, salt, and baking powder in a large mixing bowl. Add the butter and cream cheese to the bowl and use your hands to coat each piece of butter and cream cheese with flour. Then begin pressing the butter and cream cheese between your fingers to flatten each piece. Instead of making pea shapes, make flat disks with the butter, cream cheese, and flour. Continue until the mixture looks scraggly.
Pour in the egg/buttermilk mixture. Mix with your hands to incorporate the ingredients. You will need to continuously rub your hands together to keep the sticky dough from accumulating on your hands. The dough will come together into a ball in a few minutes.
If the dough is sticky, shape the dough into a disk and wrap it tightly in plastic wrap. Let it chill in the fridge for about 30 minutes or until it is no longer sticky.
The dough rolls best when it is closer to room temperature, so if you store it in the fridge or freezer for an extended period of time, let it rest wrapped in plastic for an hour or two to bring it back to room temp before rolling.
Work the dough a little with your hands to soften it up and flatten it some. Dust gluten-free flour onto your work surface, rolling pin, and the dough to prevent sticking. Roll to approximately ⅛-inch thick. Transfer to a 9-inch deep-dish pie plate, trimming and shaping edges as you like.
Preheat the oven to 375°F.
Make the crumb topping:
Mix all dry ingredients in a bowl until well combined. Add a little butter at a time until it resembles damp sand and small crumbles begin to form. Set aside.
Make the filling:
Wash, peel, core, and cut apples in even size pieces. You can slice or dice them, the shape doesn’t matter, keeping them similar size does, so that they cook at the same speed.
Put apples in a large bowl. Add the remaining filling ingredients and give it all a good stir. Make sure that all the apples are well coated with all the ingredients.
Put apple mixture into the prepared pie crust. Sprinkle the crumb topping on top of the apples.
Place on a baking sheet in the oven (to catch any delicious liquid that might escape while it bakes). Bake for 15 minutes. Turn the heat down to 350°F and continue baking for an additional 35-45 minutes or until a knife easily pierces the apples. Watch to be sure it doesn’t brown too much. If it starts getting dark, cover it loosely with aluminum foil while it finishes baking. Allow the pie to cool for about an hour at room temperature prior to serving.