For the crust: Mix salt in flour and cut butter into flour with a fork or pastry cutter until well blended. Add enough water until the dough scrapes away from the bowl - usually about 3-5 Tbsp (amount as original recipe is written // adjust if altering batch size).
Remove from bowl, mold together with hands on a well-floured surface. Form into a 1-inch thick disc and then roll out into an even circle, adding more flour if it's sticky.
Use the rolling pin to roll it up and lay it gently over a 10-inch cast iron skillet or pie baking dish and gently form it to the edges with your fingers. It should extend to the top of the pan but you don't have to crimp the edges or anything - we're going for rustic. Refrigerate until you're ready to add the apples.
For the filling: Next prep the apples. Add them to the same bowl as you made the dough in and toss with sugar, cinnamon and flour. Add to pie crust and top with 1 Tbsp butter, scored into little dots or strips (amount as original recipe is written // adjust if altering batch size).
For the topping: In the same bowl, add the crumble topping ingredients and mix with fingers or a pastry cutter until crumbly. Then pour it over the pie, spreading it evenly to coat. Bake at 400 for 35-45 minutes, or until the pie is warm and bubbly and the top is golden brown. If you notice the crumble top getting too brown, simply top it with foil.
Let rest for 30 minutes before slicing.
*Recipe (as originally written) yields 1, 10″ crust. *Crumble top adapted from my Strawberry Nectarine GF Crisp. *If you're not into the crumble top, simply double the crust recipe and lay the other half on top! *Nutrition information is a rough estimate.