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.

Preheat oven to 425 degrees F. 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. Pile in the apples. Place top crust on 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.