Cut the onion into thin strips. Salt and pepper the chopped liver and roll it in flour.
Fry the liver, stirring, in a preheated frying pan over medium heat for 3 minutes with vegetable oil. Add butter, onion and fry everything together for about 5 minutes.
Reduce heat to low and add sour cream. Simmer everything together, stirring, for 15 minutes. If the mixture is too thick, add a little water or broth.
Remove the pan from the heat and leave the dish covered for 5 minutes. Serve the beef stroganoff hot with your favorite side dish.