Technically, filet mignon is as kosher as any other cut of meat. The problem with filet mignon and other cuts from the rear is that they are located near the sciatic nerve and fatty deposits known in Hebrew as chelev, which are Biblically forbidden.1 Only a very skilled person can separate the forbidden parts 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.

Rabbi Eliezer Posner