Peel and thinly slice the apples. Reserve half of the slices for decorating the top of the cake.
Separate the eggs, beating the egg whites with a pinch of salt until stiff peaks form.
In a large bowl, use an electric mixer to beat the egg yolks, lemon zest, sugar, and a pinch of salt until light and fluffy.
Sift together the baking powder, whole wheat flour, and all-purpose flour.
Gradually add the flour mixture and milk to the egg yolk mixture in two or three batches, mixing well to avoid lumps. Stir in the lemon juice to taste, adjusting based on your desired lemony flavor.
Gently fold the beaten egg whites into the batter using an upward motion to maintain a fluffy texture.
Stir half of the apple slices into the batter. Pour the batter into a 20 cm (8-inch) round cake pan lined with parchment paper. Smooth the top, then arrange the reserved apple slices on top for decoration.
Bake in a preheated static oven at 180°C (350°F) for about 40 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool completely before removing it from the pan.