Wash, peel and cut the potatoes into pieces. Cover with cold water and put aside. Clean, clean and slice the mushrooms. Clean and wash spring onions and cut them into rings. Wash parsley, shake dry and chop except for something to garnish. Wash pork fillets and dab dry. Heat 3 tablespoons of oil in a large frying pan and fry the fillets briefly and thoroughly, turning them all round.
Season with salt and pepper, remove from the pan and place in an ovenproof dish. Add 4-5 tablespoons of oil to the hot frying fat and fry the mushrooms, turning them over. Finally mix in the spring onions, fry briefly and season with salt and pepper. Spread the mushrooms over the fillets. Put 500 g butter in a small pot and melt at low heat until it separates into three layers. (An upper thin foam layer of protein residues, the middle thick yellow butterfat and the whey which settled at the bottom). After this process, skim the foam formed at the top with a spoon and pour the concentrated butter slowly into a bowl, leaving the milky sediment in the pot
Mix the egg yolks with the cooled reduction, whisk on a moderately hot water bath until a thick, creamy foam is formed. Stir the clarified butter first drop by drop, then pour it in a thin stream into the egg yolk cream until a thick creamy sauce is formed. Add parsley and pour the herb-hollandaise sauce over the fillets and mushrooms. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 25 minutes. Meanwhile, cook the potatoes in little boiling salted water for about 20 minutes.
Heat milk. Drain potatoes, add hot milk and 30 g butter and mash coarsely with a potato masher. Season mashed potatoes with salt and nutmeg and serve with the fillet crisps. Garnish with parsley