Cut up the granny smith apples and place them int a large bowl.
Add the remaining apple filling ingredients: fresh lemon juice, brown sugar, cinnamon, salt, and corn starch. Stir to combine.
In a separate bowl, whisk together the brown sugar, oat flour, oats, and salt for the crisp topping.
Melt the butter and coconut oil together, then add them to the remaining crisp ingredients.
Use a rubber spatula to mix the crisp topping together.
Stir the apple filling again, then place it into a greased small baking dish or pie pan. Spread the crisp topping evenly over the apple filling.
Bake the gluten free apple crisp at 350 degrees for 50 - 60 minutes, or until the crisp is golden browned and the apple filling is bubbly.
Serve warm topped with vanilla ice cream.