Hanger Steak Frites with Bearnaise

Hanger steak, onglet on a French menu, hangs from the diaphragm, and every bistro butcher used to keep it for himself. It has a loose, open grain that grabs sauce and a deep, almost mineral flavor. Cook it no further than medium-rare or it tightens up. The bearnaise here is made in a blender with hot butter, which removes most of the stress; the tarragon reduction is what makes it bearnaise, so do not skip it. Double-fry the potatoes and toss them with parsley while hot.