Crumble the beef into amicrowave-proof casserole dish and add the onion.
Microwave on High for 2 1⁄2 minutes or until no longer pink, stirring once. Drain and stir to break the meat into smaller pieces. Add the tomato sauce, 400ml/14fl oz water and spaghetti.
Cover with the casserole lid. Microwave for 10–11 minutes until the spaghetti is al dente, stirring twice. Serve hot with the Parmesan sprinkled over the top