In a skillet, cook ground beef and onion in butter until the beef is browned and the onion is tender. Stir in flour,salt,garlic,pepper and mushrooms. Cook for 5 minutes, stirring constantly. Stir in soup; bring to a boil, stirring constantly. Reduce heat; simmer, uncovered for 10 minutes, stirring occasionally. Stir in sour cream; heat through, but do not boil. Meanwhile,combine noodles,poppy seed, and butter; toss lightly. Spoon stroganoff over noodles. Garnish with parsley.