Place the caster sugar, flour and eggs into a bowl and whisk together to make a thick smooth batter.
Grease and flour a deep baking dish and arrange all the apple slices to cover the bottom of the dish.
Spoon the batter mixture over the top and transfer to the oven to bake for 30-40 minutes. (To check if the sponge is ready, insert a skewer into the top of the sponge, if it comes out sticky, return to the oven to cook for another 5 minutes.).