Onglet de bœuf in French butchery refers to the hanger steak, which is a cut from the plate primal of the cow, located near the animal's belly. It is also known as "butcher's steak" because butchers would often reserve it for themselves due to its high quality and limited quantity - there's only one hanger steak per animal.
The hanger steak is known for its strong beefy flavor. It has a coarse texture compared to other steaks, yet when cooked correctly, it can be very tender. The onglet is traditionally associated with French cuisine, but it's also popular in other cuisines.