Jim Dixon told us:

Fillet – American: fil-LAY – British: FILL-it.

This word, in French, is "filet"--one L. "Fee-LAY".
As in "filet Mignon", a particular dish prepared from
a specific meat cut, not the meat cut itself, whether
raw or prepared in some other manner, which dish might
be named something like "filet Gladys" or "filet Oscar".

"Fillet", on the other hand, may be either a verb
or a noun in English.

The verb is the operation of separating the muscle tissues
of a fish (or a meat animal) from the bone(s). Pronounced
"FILL-it" or "FILL-et". The two-L word is English, not
French, and should not be pronounced "fee-LAY".

As a noun, fillet refers to either the flesh thus removed from the
bone(s) or to a physical feature making a smooth curve at the
joint of two straight lines perpendicular to each other.
("Straight" and "perpendicular" here are approximate descriptions,
of course.)

Dave Oesterreich