Parmesan is the anglicized word for parmigiano/a, from the Italian meaning from Parma.
But we pronounce the “s” like / ʐ / (“zh”). I cannot think of another English word where we pronounce an “s” like that when it is followed by “a”. (Sometimes we do for words like “cartesian”.) And Italian does not have that sound at all.
So how did we get here?
(I seem to recall a linguist here named Joanna but I do not remember her actual username, and don’t know if she’s still active here.)
@Johanna is still around and active!
My layman’s guess (happy to be corrected): Maybe we “mis-borrowed” it from French? It wouldn’t really be pronounced quite that way in French either, though.
Or treasure/pleasure/usual; the “su” is sometimes pronounced like “zhu”.
But I can’t think of another “sa” syllable pronounced like “zha”, agreed.
I am no Johanna, but I do believe it came from the French word “parmesan”, which was previously spelled “parmisan”, from “parmigiano”. And it’s not pronounced in English the way the French pronounce it, it’s just weird in English.
I suspect the spelling came from the foody-know-it-all French but the pronunciation in English was an attempt to make it sound more Italian by substituting the S with something approximating the soft G in parmigiano.
My cousin, who is a president, owns a business, where she listens to music with her husband and eats raisins. They have done this about a thousand times, until he poisened her and now she thinks it is miserable. Little did she know but this was by design where she would lose him. Little did she know that the antidote was parmesan.
Do we all pronounce it with a “zh”? I say “PAR-meg-zahn.”
I’m not sure what point you’re making, but all of the bolded S’s in your text are pronounced as a Z except the one in Parmesan which is not, at least for many (but not all) Americans.
Clearly that middle syllable was supposed to be “meh.”
I found it to be quite interesting.
Seems to non-expert me (although I’m sitting in an Italian restaurant as I post 
) that English spells it “Parmesan”, and people who only know that much pronounce it PARM-uh-Zan or occasionally -San.
If you know a bit of (restaurant) Italian you know the Italian word is Parmigiano/a. So you hyper-correct the ignorant PARM-uh-Zan to PARM-eh-ZHan with a big push on the H part of zh. And maybe finish with an o/a vowel to prove your bona fides as a food snob and language guru. 
At least that’s my technique!
That must have happened a long time ago to have been passed down to a 1960s white-bread family like mine that could barely pronounce the word spaghetti. We pronounced the last syllable like Mr. Van Damme’s first name. Even the stuff in the can.
Life is a minestrone
Served up with parmesan cheese
Death is a cold lasagne
Suspended in deep freeze
*Pronounced parm-uh-zahn in the song, therefore the definitive pronunciation.
My parents pronounced it Par me zee an
Midwest, and had never heard of ‘pasta’ until they were in there 40’s.
My history with this word is probably idiosyncratic, but I had always heard and pronounced it with the ZH sound. Then at some point I got the impression that the dish “chicken parmesan” was pronounced differently than the cheese: the dish used the ZH sound and the cheese used Z. I thought the word in the dish was some kind adjectival form, and maybe even spelled differently (“parmesian”?). So I kept the ZH sound only in “chicken parmesan” and used Z in the cheese. Eventually I realized that that idea was bogus but to this day I vacillate between Z and ZH in the word in all contexts.
There is no H part of ZH. ZH (IPA /ʒ/) is a single phoneme, the voiced version of SH (IPA /ʃ/).
10cc is an English rock band, so they spell lasagna with an “e”, though I believe it’s pronounced the same as it is in the US.
(You are in the UK if I recall.)
BTW, the OED says that “parmesan” is pronounced with a Z, not ZH, in British English.
Heh, you had me going for a long while. I was like “huh, that must be a rarer regionalism that somehow kept the g from Italian but made it hard…”
I’m a 69-year-old American. When I was a kid, everyone I knew pronounced with a “z” sound. I remember first hearing the “zh” sound in a commercial, possibly for Stouffer’s lasagna or some similar product. I was well into adulthood. At the time I took it to be a half-assed attempt to sound “authentically” Italian. My thought at the time was, “If you want to sound Italian, why not go all the way and use the word ‘parmagiana’?”
It’s parmigiano crossed with French parmesan, but with the Italian /dʒ/ replaced by French /ʒ/, producing a fairly twisted hybrid hyperforeignism, the sort of thing pretentious boozhwah English speakers keep coming up with. You know, like the ñ replacing the plain n in habanero. Vichyssoise lost its final /z/ sound. Lingerie with its bizarre pronunciation that is neither English nor French, just an I-don’t-know-what.