Preheat the oven to 350°F. Lightly grease an 8×8-inch baking dish or line it with parchment paper.
In a small bowl, mix all the topping ingredients together using a fork. Stir until the mixture becomes crumbly, then set it aside.
In a medium bowl, combine the almond flour, oat flour, salt, cinnamon, and baking soda. Stir well so everything is evenly mixed.
In a large bowl, whisk together the eggs, Greek yogurt, melted coconut oil, vanilla extract, and maple syrup until smooth.
Slowly add the dry ingredients to the wet ingredients. Stir gently until just combined. Do not overmix.
Fold in the chopped apples, making sure they are evenly spread through the batter.
Pour the batter into the prepared baking dish and smooth out the top. Sprinkle the crumb topping evenly over the batter.
Bake for 40 to 50 minutes, or until a toothpick inserted in the center comes out clean or with a few soft crumbs. After about 30 minutes, check the cake. If the topping is getting too dark, loosely cover the pan with foil to prevent over-browning.
Allow the cake to cool in the pan for 15 to 20 minutes. Then move it to a wire rack to cool completely before slicing.
Cut into squares and serve.