Chop the onions finely. Tenderize roast, season with salt and pepper and cut the edges. Heat oil and sear roast on both sides, lift from pan and keep warm.
Sauté onion, deglaze with red wine and pour in the soup. Season with 1 tbsp. mustard. Add roast joints and allow to boil over low heat until tender.
Whisk sour cream with 1 tablespoon flour and stir into the sauce 10 minutes before end of cooking.
Garnish: Peel onion and chop finely. Clean mushrooms, quarter the big pieces. Heat oil, fry onion and mushrooms, season with salt and pepper and stir in the parsley.
arranging roast beef, pour over sauce and garnish with the mushrooms.