"Fillet" or "filet"

You are correct that Filet is the French spelling of fillet. According to the Oxford online dictionary:

Filet

French spelling of fillet, used especially in the names of French or French-sounding dishes

filet de boeuf

Fillet

a fleshy boneless piece of meat from near the loins or the ribs of an animal:

a chicken breast fillet

[mass noun] :

roast fillet of lamb (also fillet steak)

a beef steak cut from the lower part of a sirloin.

a boned side of a fish.

So, when preparing a dish, as an ingredient, you should use the word fillet. When referring to the name of the dish, it will depend (as you note: filet mignon).

Additionally, the Cambridge online dictionary states that filet is the US spelling of fillet:

Filet

US for fillet

Upon asking a professional butcher in a meat market, I was informed that the word filet, pronounced 'fillay', is the process of cutting a 'fillet', pronounced 'fil'- lette'. Therefore, filet would be the verb and fillet would be the noun.

In the USA I have most often seen "filet" for the noun (regardless of whether it is fish, beef, pork, etc.) although fillet is also "permissable". It is correct either way. The English culinary word derives from the French word like many or most culinary terms such as "purée". And, for example, the interchangeable word for beef tenderloin (filet mignon). When using it as a verb (i.e. to "debone" fish or meat) it is preferred to use fillet (present tense) and filleted past tense. Example: He filleted the red snapper with his fancy Japanese steel chef's knife. Ah. By the way, a knife specially created for deboning is sometime called a "filet knife", sometimes a "fillet knife". I see them both ways on Amazon.com. And to clarify. A steak (or piece of fish) with bone removed is a filet. One with the bone is more correctly called a chop. For example, a T-Bone and Porterhouse steak are not filets, they are technically chops. No offense intended to the butcher that stated the opposite for the noun and the verb. By the way, whether spelled filet or fillet, it is pronounced "filay".

The difference in pronunciation is simple; spelling. If I see the word spelled filet on a menu I will pronounce it ‘filay’. If it is spelled fillet then it is pronounced ‘fillett’, hard t. That is because they are two separate words and should not be confused. If you wish to be pretentious by all means pronounce fillet as ‘filay’ but just know it’s wrong and people outside the US will laugh at you for it.

If it is. a french description, as in filet mignon, the t is silent, if it McDonalds' bastadised mix of English and French the t is hard!! Either fillet of fish or filet du Poisson!