1. Preheat the oven to 200C/400°F.
2. Cover a baking sheet with parchment paper. Place the walnuts on it and bake. Let the walnuts brown for 5 minutes. Remove from the oven and set aside.
3. Lower the oven temperature to 180C/350°F.
4. In a large bowl, using an electric whisk, whisk the sugar and butter until they are foamy.
5. Add the eggs, one at a time, and mix after each addition.
6. Add the flour and baking powder, and mix gently.
7. Add the diced apples and roasted walnuts. Mix.
8. Grease and flour a cake mold. Pour the cake batter into it.
9. Bake for 35 minutes.
10. Pierce the center with the tip of a knife. If the knife comes out clean, remove the cake.
11. Place the cake on a rack and let it cool for 5 minutes before unmolding. Serve warm or cold