Preheat the oven to 180°C (350°F).
Grease a baking dish with butter.
Peel and core the apples, then cut them into thin slices.
In a bowl, mix the apple slices with 1 tablespoon of sugar and the ground cinnamon. Set aside.
In another bowl, whisk together the eggs and remaining sugar until light and fluffy.
Add the vegetable oil and vanilla extract, continue to whisk until well combined.
In a separate bowl, sift together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until fully incorporated.
Fold in the chopped walnuts.
Pour half of the batter into the prepared baking dish.
Layer the apple slices evenly over the batter.
Pour the remaining batter over the apple layer.
Bake for 40-45 minutes or until a toothpick inserted into the center comes out clean.
Let it cool before serving.