In a large mixing bowl, crack the 3 eggs, then add 100g of zero-calorie sweetener. Whisk until the sugar dissolves. Pour in 80g of melted butter, continuing to whisk until fully combined. Gradually add 350g of milk, mixing as you go to ensure all the wet ingredients are well incorporated. Finally, add 9g of baking powder and mix until smooth.

In another bowl, combine 250g of cake flour, 50g of oats, and 50g of flaxseed powder. Stir to evenly distribute the dry ingredients.

Slowly add the dry mixture to the wet mixture, in batches. After each addition, use a spatula to fold the ingredients together gently. Be careful not to over-mix to maintain a soft texture. Stir until no dry flour is visible, and the batter becomes smooth. Let it sit for 15-20 minutes to allow the ingredients to fully meld.

While the batter rests, wash and peel the apples. Core them and cut them into even cubes.

Add 15g of ground cinnamon to the batter and mix well. Gently fold in the diced apples.

Grease a suitable cake pan with butter or line it with parchment paper to make removal easier. Pour the batter into the pan, smoothing the top.

Sprinkle walnuts on top of the cake for decoration. Preheat the oven to 160°C (320°F). Once the oven is ready, place the cake pan in the center and bake for 50 minutes. Keep an eye on the cake as it bakes—if the top begins to brown too quickly, cover it with aluminum foil to prevent burning.

After 50 minutes, insert a toothpick into the center of the cake. If it comes out clean, the cake is ready. Remove the cake from the oven and let it cool on a wire rack for a few minutes. Once cooled slightly, remove it from the pan and cut it into slices.