Peel and wash the potatoes and cook them in boiling salted water for 20 minutes. Peel onions and cut into rings. Wash and quarter the apple and remove the core. Cut the apple quarters into small pieces.
Heat the fat in a pan and sauté the apple pieces in it. Heat the oil in a second pan and fry the onion rings in it, remove and keep warm. Fry the black pudding slices in the frying fat on both sides for two minutes each.
Drain and mash the potatoes or press them through a potato ricer. Warm milk and stir in. Season the mashed potatoes with salt, pepper and nutmeg and fold in the apple pieces. Arrange mashed potatoes, black pudding and onion rings on plates and serve garnished with parsley and apple slices.