Brown ground beef and chopped onion. Place in a Dutch oven or large fry pan with sliced mushrooms and melted butter and cook until mushrooms are tender. Stir in flour and next 2 ingredients; cook and stir 1 minute. Stir in soup. Simmer 10 minutes, stirring occasionally. Add sour cream, and heat thoroughly. Serve over noodles. Serve over cooked egg noodles.