Where, if anywhere, do commas belong in a "not only... but" sentence in which the "but" clause appears in mid-sentence?
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.
Edwin Ashworth's advice in a comment above is very sound. There is no standing rule requiring writers to split up a "not only ... but also ..." construction with a comma—regardless of where that construction falls in a sentence—and in many instances the sentence will be better off without the extra punctuation.
The only categorical exception to this recommendation (again as Edwin says) is when the components of the "not only" and "but also" components are so long that readers may lose sight of the controlling structure without a comma to signal the break between the "not only" piece and the "but also" piece.
So of the OP's three options, I think that the first is the most sensibly punctuated:
Not only cheddar cheese but also mozzarella cheese is really good on Triscuits.
If you don't like the syntactical awkwardness of that sentence, however, you should feel free to switch to the simpler wording endorsed in user114275's answer:
Both cheddar cheese and mozzarella are really good on Triscuits.
Not only but also is a correlative conjunction. When you use this conjunction (regardless of its location), you do not use commas. For example, I not only like you but also respect you. She is not only a teacher but also a good photographer. You like not only cheddar cheese but also mozzarella cheese.
Your last choice is best as it sets the phrase apart as a nonessential element, therefore, needing commas. Correlative conjunctions tend to be formulaic, so if you don't like your book example, I would rewrite the sentence because it's awkward. You might consider something like the following: "Both mozzarella and cheddar cheese are good on Triscuits."
Commas are typographic elements and as such should appear anywhere that helps convey the meaning of prose or the patterns of quoted speech; no other rule matters. In this case the comma doesn't make much difference in the second example and the two are slightly confusing (IMO) in the third. In other words you don't really need a comma here.