Wash the potatoes thoroughly, peel them, wash them again and cook them in boiling salted water for 25 minutes. Meanwhile wash, peel and quarter apples, cut out the core. Cut apples into cubes. Bring apple juice, lemon juice, vanillin sugar and apple cubes to the boil and simmer for ten minutes. Peel onions and cut into rings. Heat oil in a pan and fry the onion rings in it until golden brown. Heat the milk and half of the fat.
Drain the potatoes, add the milk and mash with a potato masher. Season to taste with salt and nutmeg. Melt remaining fat and stir in breadcrumbs. Put potato puree and apple compote in a greased casserole dish, place black pudding on top and spread breadcrumbs and onion rings over it. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 15 minutes. Wash parsley, dab dry and chop except for a little bit for garnishing. Serve sprinkled with chopped parsley and garnished