Cook the beef and onion in a 12 inch skillet until the beef is well browned, stirring frequently to break up meat. Pour off any fat. Spoon the beef mixture into a 13x9x2 inch(3 qrt) shallow baking dish. Stir the soup, noodles, water, carrots and sour cream into the dish. cover.
Bake at 375 degree F for 30 mins or until hot and bubbly.