In a large skillet, brown and crumble the ground sirloin with the chopped onion until no longer pink; drain excess fat, if desired (there shouldn't be that much if you've used the extra lean ground sirloin).
Return cooked beef/onions to skillet. Add the tomato sauce, water, diced tomatoes, dry macaroni and seasonings. Stir well and bring to a boil.
Reduce heat to medium-low. Cover tightly and simmer for 12-15 minutes (stirring once or twice) or until macaroni is tender.
Add milk and cheese and stir well, until cheese is smooth and melted.