Crumble ground beef into a 1-quart microwave-safe casserole; add onion. Microwave on high for 2 2 1/2 minutes or until no longer pink; stirring once. Drain; stir to break meat into smaller pieces. Add tomato sauce, water, and spaghetti. Cover with casserole lid. Microwave 10 11 minutes or until spaghetti is tender, stirring twice. Serve with Parmesan Cheese.