Stir butter until foamy, mix with flour, baking powder and salt and knead briefly. Add vinegar, water and nuts and knead into a smooth dough. Place in the fridge for 30 minutes, wrapped in cling film.
Covering:
Cover the leeks and onions in heated oil and 4 tablespoons of water over a low heat for 15-20 minutes. Then add the potatoes and paprika pulp and cook for another 5 minutes.
Add half of the cheese, marjoram, nutmeg, salt and pepper and stir well.
Roll out the dough and line a tart pan with it. Place the topping on top and sprinkle with the rest of the cheese.
Bake in a preheated oven at 180 degrees for about 35 minutes.