Melt 3 tablespoons of the butter in the oil in a heavy 10- to 12-inch skillet over moderate heat. Add a few of the beef slices, and fry them for two minutes on each side or until lightly browned.
With tongs or a slotted spoon, transfer the slices to a plate. Fry the remaining slices in batches that do not crowd the skillet, add more butter if necessary.
Melt two tablespoons of butter in the skillet and stir in the scallions and mushrooms. Stirring frequently cook for five or six minutes or until they are soft and lightly browned. Add the flour and mix thoroughly with the vegetables, then pour in the white wine and add the sour cream, 1 tablespoon at a time. Stir in the tomato paste, Worcestershire sauce, salt and a few grindings of black pepper.
Return the beef to the pan, together with the juices that have accumulated on the plate. Coat the meat thoroughly with the sauce, cover the pan, and cook over very low heat for two to three minutes or until the beef is heated through. Taste for seasoning and transfer the beef stroganoff to a large heated platter. Sprinkle with the parsley and serve at once.