Preheat your oven to 350° and grease a 8 x 8 baking dish.
Make the topping: pulse the walnuts in a food processor 10-12 times or until they are course crumbs. Add the remaining ingredients and pulse 2-3 more times until combined. Set aside.
Wipe out and dry the bowl of your food processor and add your dry cakeingredients. (almond flour through salt) Pulse a few times to mix.
Cut the tablespoon of butter into smaller chunks and add it to the dry ingredients. Pulse 8-10 times or until it’s cut in to the dry ingredients, similar to if you were making a pie crust.
In a small bowl, mix your wet cake ingredients (eggs through vanilla) and whisk until well combined. Stir in grated apple.
Add to the food processor and mix until combined. Scrape down the sides once or twice to make sure it’s well mixed.
Pour into the prepared baking dish and sprinkle the topping over, as evenly as you can.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow to cool, and enjoy!