Preheat & Prepare the Pan: Preheat your oven to 180°C (350°F). Line a 9-inch (23cm) springform pan with parchment paper and set aside.

Make Oat Flour (if needed): If you're not using store-bought oat flour, measure out 4 cups (360g) of rolled oats, then place them in a blender or food processor. Blend until a coarse flour forms. It doesn't need to be perfectly fine — a little texture is great! Note: You can also combine different types of oats (like quick oats, oat flour, or whole rolled oats) for a layered texture — see notes below.

Make the Oat Mixture: In a large bowl, combine the oat flour, sugar, and cinnamon. Mix well to evenly distribute the dry ingredients.

Grate the Apples: Peel and grate the apples using a box grater. Place the shredded apples in a bowl and mix with fresh lemon juice to prevent browning (you will need about 1 KG of grated apples). Set aside to release some juices.

Layer the Cake: Start by grating about 2 tablespoons (30g) of cold butter directly into the bottom of the prepared pan. Sprinkle 3–4 tablespoons of the oat mixture over the butter, pressing it down gently to form the base layer.

Spread ⅓ of the grated apples (along with some of the juice) evenly over the oats. Repeat the layering process two more times: oat mixture → apples → oat mixture → apples. Finish with the remaining oat mixture, then grate the remaining 125g of cold butter over the top. Sprinkle with the remaining 1 tablespoon of sugar for a golden, slightly caramelized crust.

Bake: Bake in the preheated oven for 35–45 minutes, or until the top is golden brown and the apples are soft and bubbling.

Cool & Serve: Let the cake cool in the pan for about 20 minutes, then carefully release the springform and allow it to cool completely before slicing. It’s best served at room temperature or slightly warm.