Cut the meat across the grain into thick strips and fry for 20 minutes in a preheated frying pan with oil. Cut the onion into half rings and add to the meat, fry for 5 minutes. Cut the champignons into slices and also add to the meat, cook for another 10 minutes.
Add salt and pepper to taste to the meat, onions and mushrooms in the frying pan, then add sour cream, mix everything, cover with a lid and leave to simmer for 10 minutes.