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 pat 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 while turning. Finally mix in the spring onions, fry briefly and season with salt and pepper.
Spread the mushrooms over the fillets. Put 1/4 litre of cold water into a pot, first stir in the sauce powder with a whisk then stir in the cream. Bring the sauce to the boil while stirring and turn off the hotplate.
Cut 120 g cold butter into cubes and stir into the sauce in portions. 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.