Preheat your oven to 350°F.
In a mixing bowl or food processor, combine 1 cup oat flour, 1/3 cup tapioca flour, 1/4 teaspoon salt, and 1/4 teaspoon cinnamon. Pulse until blended. Cut 1/2 cup butter into small pieces and 2 tablespoons maple syrup into the mixture and pulse until the mixture is crumbly. Add 1 tablespoon cold water and pulse until a dough forms.
You can either press the dough into your pie pan by hand or roll the dough out on parchment paper with a little extra tapioca flour. Then, place the dough inside the pie pan and shape it as desired. Place the pie crust in the fridge while you prep the filling.
Core and thinly slice 4 large apples, 5 medium apples, or 6 small apples. Peeling the apples first is optional. I like to use a mandoline slicer and then cut the thin apple rings into fourths.Toss the apples in a bowl with 1/3 cup coconut sugar, 1/4 cup maple syrup, 1 teaspoon cinnamon, 1/4 teaspoon nutmeg, and 4 teaspoons cornstarch.
In a bowl, stir together 3 tablespoons melted butter, 3 tablespoons maple syrup, 1/2 cup oats, 1/4 cup oat flour, 1/3 cup chopped pecans, 1/4 teaspoon cinnamon, and 1/4 teaspoon salt.
Spread the apple slices in the pie crust. Arrange them so they are all horizontal and fill in any cracks. Pour any additional liquid from the bottom of the bowl over the apple slices in the pie.Sprinkle the crumble evenly over the apple filling. I like to leave a 1" gap between the crumble and the edge of the pie so the apple slices are visible. Cover the pie loosely with aluminum foil and bake for 30 minutes. Remove the foil and bake for another 20-25 minutes, until the topping is golden and the filling is bubbling. Allow the pie to cool completely on the counter or in the fridge before slicing and serving.