Heat the oven to 350 degrees. Oil or butter an 8- or 9-inch loaf pan, then line the pan with parchment paper, letting excess paper hang over the sides. Don’t worry about getting neat creases with the paper; this is a casual, rustic-looking cake.
Peel, core and halve the apples, then slice them very thinly, about ⅛-inch. You can use a mandoline for this, or just proceed carefully with a sharp knife. Put the apple slices in a large bowl and toss them gently with the juice from the lemon.
In a small bowl, combine the flour, baking powder, cinnamon and a pinch of salt.
In another bowl, using an electric mixer or by hand, beat the eggs with the sugar until the mixture is thoroughly combined and voluminous. Add the dry ingredients a little at a time, mixing between each addition. Add the melted butter and the milk and mix to combine.
Add the cake batter to the bowl with the apples, mixing gently to avoid breaking them (though some broken slices are totally OK). Transfer the apple mixture to the prepared loaf pan, gently smoothing out the mixture as you go to make sure the apple slices are stacking and lying flat. Bake until a cake tester or toothpick inserted into the cake comes out clean, about 1 hour.
Let cake cool completely in the pan, about 3 hours. Use parchment paper to lift out cake, then slice.
