Wash and core the apples. Slice them thinly (2-3mm thick).
Make the cake mix by mixing all dry ingredients together, then add egg, yogurt and oil. Mix together until you get a sticky thick dough. If using, make the crumble by rubbing all the flour, icing sugar and butter until you get a crumbly texture. Set aside.
Prepare the baking tin: make it non-stick by rubbing butter around all of the insides and dusting it with semolina or all-purpose flour.
Pour the cake mix into the baking tin, push it all to the sides (use a spatula), top the dough with sliced apples and the crumble, if using.
Bake for 1 hour at the lower third of your oven until golden. Check with a bamboo stick that it comes out clean, if not, bake it for another 5 mins.