Peel and wash the potatoes and cook them in boiling salted water for 20 minutes. Peel onions and cut into rings. Peel and quarter apples, except one, and remove the core. Cut the apple quarters into pieces.
Wash the remaining apple, quarter it and remove the core. Cut quarter into pieces as well. Drizzle apple pieces with lemon juice. Bring apple juice and apple pieces to the boil in a saucepan and boil for five minutes.
Stir in sugar. Heat the oil in a pan and fry the onion rings in it, remove and keep warm. Halve the black pudding slices and fry them in the frying fat for two minutes on each side. Drain and mash the potatoes or press them through a potato ricer.
Warm the milk and stir it in. Season mashed potatoes with salt, pepper and nutmeg. Arrange mashed potatoes, black pudding, apple compote and onion rings on plates and serve garnished with parsley and apple slices.