Preheat your oven to 350 F.
In a large bowl, mix together the rolled oats, oat flour, walnuts, cinnamon, nutmeg, and salt.
Add in the soft coconut oil and maple syrup and mix together until a crumble forms.
Line a 9 x 13 baking dish with parchment paper. Flatten ⅔ of the crumble mixture into the bottom of the dish, reserving the rest for the top. Set it aside while you prep your apples.
In a medium saucepan over medium heat, toss together the sliced apples and all filling ingredients. Mix together over a low/medium flame for 10 minutes, until the apples begin to soften.
Layer the apple mixture over the base, and cover with remaining crumble. Flatten it down gently so the top is even.
Bake for 30 minutes. Let cool for at least 20 minutes before slicing.