Wash the fillet and dab dry. Cut celery and carrots into slices. Cook potatoes in boiling salted water for about 20 minutes
Heat 2 tablespoons of oil and fry the fillet all around. Deglaze with 1/4 litre of vegetable stock and braise in the closed pan at medium heat for about 15 minutes
Heat the remaining oil. Sauté the mushrooms, carrots and celery in it. Deglaze with remaining vegetable stock and simmer for about 10 minutes with the lid closed. Season with salt and pepper. Wash parsley, dab dry and pluck. Mix into the vegetables
Drain meat stock and mix with sour cream. Thicken slightly, bring to the boil briefly. Season fillet with salt and pepper, cut open. Serve with vegetables, boiled potatoes and sauce