To prepare the rhubarb, clean, peel and cut into pieces approx. 2-3 cm long. Put aside.
For the crumble, sieve the flour into a bowl, mix with the oat flakes and sugar. Add the softened butter in pieces and first crumble it finely with the hand mixer on the highest setting, then use the lowest setting to make crumbles of the desired size. Refrigerate.
Grease a springform pan and chill. Heat the oven to 200 ° C.
For the dough, stir the butter with the sugar until foamy, stir in the eggs one by one. Sift in the flour and baking powder and stir in.
Pour the dough into the prepared form and smooth it out.
Spread the rhubarb on top.
Bake in the preheated oven for about 15-20 minutes.
Meanwhile, beat the eggs with the sugar until creamy. Stir in the quark. Peel, quarter and core the apple and cut into fine wedges.
Spread the quark mixture on the cake and cover with apple wedges. Spread the crumbles evenly on top.
Bake for another 30-35 minutes at the same temperature.