The process of removal of the GID and other vessels is known as "Nikur" or "treibering". It is, in most cases, not economically feasible to remove the "gid". The whole hind section of the animal is generally sold as nonkosher (thereby removing the GID from the rest of the cow’s body). This is the standard practice in the U.S. In Israel, however, where there is not a so readily available nonkosher market, nikur is done. This means that some fancy hind cuts of meat are available to the kosher consumer. (Tenderloin, sirloin, porterhouse...)

Technically, filet mignon is as kosher as any other cut of meat. The problem with filet mignon is that it is located near the sciatic nerve, which is Biblically forbidden.1 Only a very skilled person can separate the forbidden nerve from the nearby kosher meat, in a process which is called "nikkur" ("tunneling, deveining"). Thus it's hard to find kosher filet mignon, especially in the United States, where I believe most slaughterhouses sell the hind portions of the cattle to non-kosher butchers -- but it does exist.

The same is true with rump and sirloin steaks, and leg of lamb.

So you think I'm really getting filet mignon? I doubt it.

Can a T-bone be trabored?

To conclude: Technically, the only forbidden parts of a cow would be specific fats, membranes, cords, veins, blood and the Gid Hanoshe.

Given that broad a range, one would technically be permitted to eat any cut of meat when its forbidden parts have been excised. In practice however, in the USA, no meat from the animal’s hindquarters is sold as Kosher.*

Don't high scale restaurants like Las Marais serve Filet Mignon???

I have not seen a filet mignon on a menu, I saw it at the supermarket.
I have seen Tenderloin steaks on numerous menus. I last saw it at Noah's Ark.

I did eat recently at Glatt-ala-cart (food was outstanding despite the corny name) and they served a filet something-else.

They've got Filet au Poivre listed on their website.

They've got Filet au Poivre listed on their website. "Filet" just means a boneless piece of something. You can have a filet of fish, too, after all.

Wolf & Lamb have a filet mignon on their menu.