Place the mixed berries in a large bowl. Add the tapioca flour, coconut sugar, cinnamon, and vanilla extract. Toss to coat all of the berries and pour into the prepared pan.
For the crumble topping, combine the almond flour, coconut flour, cinnamon, rolled oats, maple syrup, coconut oil, and walnuts in a mixing bowl. Stir until the dry ingredients are completely moistened and then crumble evenly over the berries.