Pierce the potatoes a few times with a fork, then microwave until soft, about 15 minutes.
Meanwhile, pierce both sides of the steak with a fork. Mix the mustard and herbes de Provence in a bowl and rub all over the steak.
Heat a large cast-iron skillet over medium-high heat. Add 1 tablespoon butter; as soon as it melts, add the steak and sear on one side until browned, about 7 minutes. Turn and brown the other side, about 4 more minutes for medium-rare. Transfer the steak to a cutting board, season with salt and pepper, and top with 1 tablespoon butter. Let rest at least 5 minutes.
Return the skillet to medium-high heat. Add the potatoes and turn to coat on all sides with the pan juices. Cook until the skins are slightly crisp, about 3 minutes.
Mix the remaining 3 tablespoons butter with the chives and season with salt. Thinly slice the steak on the bias. Serve with the potatoes, chive butter and mustard.
Photograph by Antonis Achilleos
Courtesy of Food Network Magazine