Boil the cream and 1/8 l water with the vegetable stock in a saucepan, add the potatoes and simmer for about 10 minutes. Season with salt, pepper and nutmeg.
Peel and dice the onions, clean and halve the mushrooms.
Heat oil in a pan. Steam the onions until translucent. Add and sauté the mushrooms, then remove and set aside.
Brown the meat for about 4 minutes, season with salt and pepper.
Finely grate the cheese. Put the potatoes in a baking dish. Scatter cheese on top. Cut the fillet into slices about 2 cm thick. Spread the mushroom and onion mixture and the meat on the potatoes.
Bake the gratin for 20 minutes.
Finely chop the parsley and tempt with the crème fraîche, salt and pepper, spread on the gratin before serving.