In a skillet, cook onion and mushrooms in olive oil over medium heat for about 10 minutes. Add flour and cook an additional minute. Drain roast beef, reserving 1 cup of the beef broth. Whisk the beef broth with the mustard, cream and sour cream. Add to skillet and continue to heat until thickened. Add the beef, parsley, salt and pepper. Cook until warmed through, about 5 minutes. Serve over cooked egg noodles. Top with parsley.