For the quark oil dough, mix the flour with the baking powder. Add all the ingredients and mix with a mixer with dough hook first at the lowest, then at the highest level briefly to a smooth dough.
(Do not knead for too long, otherwise the dough will stick!) Form into a ball, wrap in cling film and chill for about 60 minutes.
Preheat the oven to 180 °C top and bottom heat. Grease the round baking tray with butter.
For the filling, peel, halve and chop the onions. Clean, wash and chop the spring onions. Heat the oil in a frying pan and fry the onions in it until transparent.
Mix crème fraîche, egg, milk and starch in a mixing bowl and fold into the onion strips. Stir in ham and season well with salt and pepper.
Roll out the dough on a lightly floured work surface slightly larger than the form. Place it in the mould so that the edge is also covered with dough. Spread the onion filling on the cake base and bake in the oven on the grill on the middle shelf for about 50 minutes.
After the baking time, let the onion cake stand for about 10 minutes, then remove from the tin and cut. Tastes hot or cold.