Preheat the oven to 170°C and grease a 23cm round cake tin.
Coat the apple wedges in lemon juice and set them aside.
Beat the butter and caster sugar together until pale and fluffy.
Add the eggs and beat the mixture until smooth.
Sift in the flour, baking powder, and salt, then mix well.
Gradually stir in the milk until the batter is smooth.
Pour the batter into the tin and arrange apple slices on top in a spiral.
Combine the demerara sugar and cinnamon and sprinkle over the cake.
Bake for 40 to 45 minutes until golden-brown and a skewer comes out clean.
Cool in the tin for 15 minutes before transferring to a wire rack.