Cut the beef fillet into strips. Skin the onions, halve them and cut them into rings. Clean and slice the mushrooms, peel the tomatoes. Heat the oil in a large frying pan and fry the fillet strips in it. Season with salt and pepper, remove from the frying pan and keep warm.
Put the rest of the oil in a dish and heat it together with the butter. Sauté the half onion rings in it until soft.
Now add the mushrooms and tomatoes. Stir everything together heartily and season. Then fold in the sweet whipped cream and cook lightly. Stir the sour cream with the mustard and the juice of a lemon and add. After the liquid has formed foam, sprinkle the flour over it. Beat everything together until hearty and finally pour in the meat.
Serve with long-grain rice, spareribs or potatoes, depending on your taste.