Dry meat with paper towels. Cut across the grain into 1cm slices and tenderize on both sides; then cut into 5 mm strips. Peel the onion and cut into thin rings. Remove the champignon stipes and caps and cut into plates. Heat the butter in a deep frying pan and fry the meat and onion slices for 3-4 minutes, stirring constantly. Add flour, stir and heat for 1 minute. Place the meat in any container suitable for the steamer, add chopped mushrooms and sour cream. Season with salt and pepper. Cook for 25 minutes with intense boiling.