In a large bowl combine the flour, salt and pepper. Toss the beef strips in the mixture until they are coated. Shake off any excess flour. Put the strips aside.
Heat the butter in a large, heavy frying pan over moderately high heat until bubbly. Add half the meat and cook, stirring once or twice, for 1 – 2 minutes or until no pink remains. Transfer the cooked meat to a plate. Cook the remaining meat in the same way and add to the plate.
Reduce the heat to moderately low. Add the onion and garlic and cook uncovered for 5 minutes, or until the onion is soft. Add the mushrooms, beef stock and tomato paste, raise the heat to high, and cook for 2 minutes, scraping up any brown bits stuck to the pan.
Return the meat and any accumulated juices to the pan and heat through – about 1 minute. Remove the pan from the heat and stir in the sour cream (or yogurt). Serve immediately over buttered noodles or rice, and with a lettuce and tomato salad.