Fillet or Filet

Fillet is the more general term for a strip of boneless meat, and filet is usually reserved for French cuisine and in the names of French-derived dishes such as filet mignon. The Cambridge online dictionary states that filet is the US spelling of fillet:

Fillet [ˈfɪl.ɪt] ‘fill-it’

Fillet is both a noun and a verb. Filleting refers to the process of cutting meat or fish off of the bone, and the resulting piece of boneless meat or fish was historically called a fillet. The cut of meat known as a chicken tender is referred to as a chicken fillet.

Filet [ˈfɪl.eɪ] [fɪˈleɪ] ‘fillay’

Filet is the alternative French spelling of fillet. The common usage of filet in the US is for a piece of steak cut from the beef tenderloin or other expensive cut of meat. Because of this usage, filet is used more widely in reference to meat than fish.

McDonald’s Filet-o-Fish

How should you pronounce the McDonald’s Filet-o-Fish? The official McDonald’s answer is:

‘“Filet-o-Fish” can be pronounced any way you wish. Most people say “Filay”.’

Fillet ‘O’ Fish clip from Jack Dee’s 1994 Stand-Up show