1. Start by preparing the topping. Chop the walnuts (or pecans) coarsely. Add the nuts with sugar, flour, ground cinnamon, and melted butter to a bowl and mix until well combined. Set aside.

2. Preheat the oven to 350°F (180°C). Line a 9×5-inch loaf pan* with some baking parchment and grease lightly. Quarter and core the apples and cut them into small pieces. If you prefer »skinny« apples, you should peel them first. Set aside.

3. Add the sugar (or xylitol) and eggs to a large bowl and mix until very light and fluffy. Add the Greek yogurt, apple sauce, and vanilla extract and mix in. Next, add the melted butter and mix until well combined. In a separate bowl mix both types of flour, baking powder, apple pie spice, and salt. Add to the large bowl and fold in. Add the diced apples and fold in as well. Transfer about half of the batter to the prepared loaf pan and sprinkle with some of the topping. Next, spread the remaining batter evenly on top and sprinkle with the rest of the topping.

4. Press the topping slightly into the batter and bake the cake for about 60-65 minutes or until a wooden skewer inserted into the center of the cake comes out clean. Take out of the oven and let cool down inside the pan for some time, then lift out with the baking parchment and let cool down completely on a wire rack. Decorate the cooled cake with sugar glaze to your liking (optional).