In a large bowl, whisk together the oat flour, coconut sugar, and salt. Cut in the butter with a fork or your hands until the flour is crumbly and the butter is well integrated. Using a fork, stir in the water a little bit at a time (you may not need all of it) until the dough holds together. Knead it with your hands until it can form a smooth ball, then divide the ball in half, press each half into a disk, wrap in plastic wrap, and rest in the fridge at least a half hour.
Make Pie
Preheat oven to 425℉. Rub a pie pan with butter and have a baking sheet ready.
Whisk together the coconut sugar, cinnamon, nutmeg, and salt in a small bowl, then mix lightly through the apples.
Roll out half of the pie crust and fit it into the pan. You may need to use your hands to help press it in. Pile in the apples. Roll out the remaining dough and place the top crust on the pie, crimp the edges, and cut a few vents in the top.
Place pie on a baking sheet, place in the oven and bake for about 50-60 minutes or until the crust is lightly browned, apples are cooked through, and the filling is bubbling.
Remove pie from the oven, place on a wire rack, and let cool completely, at least 2 hours, so the filling can set. Serve warm or at room temperature. Pie will keep, covered in the fridge, for up to 5 days.