Heat the oil in a large frying pan and brown the fillet cut into strips. Season with salt and pepper, remove from the pan and keep warm.
Pour the remaining fat into a dish and heat it together with the butter. Fry the onions, which have been cut into half rings, and add the sliced mushrooms and the chopped tomatoes. Stir well and season. Pour on sweet whipped cream and boil gently. Mix sour cream with mustard and juice of one lemon, add and – after the liquid has foamed up – sprinkle flour over it. Beat vigorously, add the meat and, once it is well heated, serve with potatoes or long grain rice.
Personally, I find sparrows the best with it.