Mix all dry ingredients together.
Peel the banana and mash it with a fork.
Mix together with the mineral water and dry ingredients to form a smooth dough.
Preheat the oven to 180 degrees Celsius top and bottom heat.
Line a cake pan with parchment paper and pour in the batter.
Wash and peel the apples and cut them into thin slices. Arrange these on top of the cake.
Bake in the oven at 180 degrees for 20 minutes.
In the meantime, prepare the quark mixture. To do this, mix all the ingredients together in a bowl. Finally, sprinkle in the locust bean gum and stir in quickly. The mixture thickens very quickly due to the locust bean gum.
After baking for 20 minutes, remove the cake and spread the quark mixture on top. Then bake for another 20 minutes in the oven at 180 degrees.