Half, core and thinly slice apples and transfer to a large mixing bowl.
Add the cinnamon, vanilla extract, applesauce, cornstarch and maple syrup (if using sweetener), and toss until apples are evenly coated.
Transfer the apples into a baking dish, cover with foil and pre-bake at 350F (180C) for 20 minutes.
While apples are baking, in a bowl, add the rolled oats, ground oats, finely chopped walnuts, cinnamon, maple syrup and coconut oil. Using a fork mix to combine.
Remove the foil, using a spoon stir the apples, sprinkle the oat topping all over (but do not press down), and place back in the oven.
Bake at 350F (180C) for another 20-25 minutes, or until the topping is golden brown.
Let it cool down for 15 minutes, then serve with a spoonful of Greek yogurt or coconut whipped cream on top.