Peel and wash the potatoes, cover and cook in salted water for about 25 minutes. Peel onions and cut into rings. Dab fillet dry. Put in a freezer bag and tap flatter
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2). Heat 1 tablespoon of oil in a roaster. Sauté the fillet for approx. 2 minutes, turning it over. Season with salt and pepper. Add onions and fry for 2-3 minutes. Dust onions with flour and sauté briefly until golden brown. Stir in sour cream and 5-6 tbsp. water. Cover and braise in the oven for about 20 minutes.
For the salad, peel and finely dice the shallot. Wash the parsley, shake dry and cut the leaves into strips. Season vinegar with salt, pepper and 1 pinch of sugar, fold in 2 tablespoons of oil. Stir in shallot and half of the parsley. Clean and wash the lettuce hearts, spin dry and pluck into small pieces. Mix with vinaigrette
Drain the potatoes. Add cream and mash finely with a potato masher. Season to taste with salt and nutmeg. Melt butter in it
Lift out the fillet. Stir wine into the onion sauce. Season to taste with salt and pepper. Cut the fillet open. Serve with sauce and mashed potatoes. Sprinkle the rest of the parsley over it, add the salad