The fillet is a cut of beef, generally from cattle, but also from pigs and other animals including hare or deer and is taken from the lumbar of the animal. The term fillet can also be used for other, more tender parts of chicken (and other edible birds), notably from around the hull, or sternum crest (protruding cartilage) of the chest, as well as from the boneless thigh, which is known as thigh fillet.
Poultry fillet in terms of poultry, is also a term that can be used for ‘magret’, a French dish that also has roots in northern Spain, that consists of duck breast or fattened goat cooked in blood or smoked.
The cut of fillet affect the large front psoas muscle, up to the lumbar vertebra near the kidneys. Since this muscle is rarely used during the lifetime of the animal, it is an extremely tender piece of meat. Fillet can be used from roasts, to steaks, and the Fiorentina steak can include both thread and sirloin.