In a 3 quart microwave safe casserole, crumble beef and add onions. Microwave on high for 3-4 minutes, stirring to break up meat. Microwave on high for 2 more minutes and drain off fat.
Add spaghetti sauce, water, basil and uncooked spaghetti. Mix to combine. Cover with lid and microwave on high for 5 minutes. Stir. Cover casserole again and microwave on high for 8-10 minutes longer. Stir.
If you want to freeze this dish, at this point cool the casserole in the refrigerator. Wrap, label, and attach a freezer bag with the grated Parmesan cheese. Freeze up to 3 months. To thaw and reheat, thaw overnight in refrigerator. Cover casserole with microwave safe paper towel and microwave on medium for 7-8 minutes until warm. Sprinkle with cheese and continue as directed below.
If not freezing, sprinkle with cheese, cover again and microwave on high for 7-8 minutes longer until spaghetti is tender. Let stand 4 minutes before serving.