In a shallow dish, combine the flour, salt, paprika and pepper. Add the beef strips, then turn or toss until they're well coated. Editor's Tip: Putting the meat in the freezer for 20 minutes or so helps firm its texture and makes it easier to cut into thin, even strips. In a large skillet, melt the butter. Brown the beef in batches, then remove it and keep it warm. Add the onion and garlic to the pan, and cook them until tender. Add the broth, mustard, chili sauce and mushrooms, then return the beef to the skillet. Cover and cook until the meat is tender, about one hour. Just before serving, stir in the sour cream. Heat through, but do not boil. Serve the beef mixture over noodles, garnished with parsley.