I tried to find this rule explicitly on several grammar sites, but did not find it.
A "not only" sentence does not normally need a comma as the "not only" and "but also" are correlative pairs. So "Not only does he like cheddar cheese but also mozzarella" does not need a comma.
My question is what happens if the "but also" clause is in the middle of the sentence, such as:
Not only cheddar cheese but also mozzarella cheese is really good on Triscuits.
Should there be a comma after "cheddar cheese" like so
Not only cheddar cheese, but also mozzarella cheese is really good on Triscuits
or maybe also after "mozzarella cheese" like so
Not only cheddar cheese, but also mozzarella cheese, is really good on Triscuits.
I'm not sure which way seems to leave either clause less incomplete.