Break the raw ground beef into large pieces in a 4- to 6-quart slow cooker. Scatter the finely chopped onion over it.
Pour in the undrained diced tomatoes and tomato sauce. Cover and cook on Low for 4 to 5 hours or High for 2 1/2 to 3 hours, until the beef is fully cooked and the onion is soft.
Stir well to break the beef into small crumbles. Check that the beef has reached at least 160°F (71°C) in the thickest pieces.
Stir in the uncooked elbow macaroni so it is mostly submerged in the hot tomato mixture.
Cover and cook on High for 20 to 30 minutes, stirring once if needed, until the pasta is tender. Add a splash of water if the mixture is too thick.
Spoon the goulash into bowls and serve hot.