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.
Per serving: Calories 478; Fat 20 g (Saturated 11 g); Cholesterol 99 mg; Sodium 217 mg; Carbohydrate 31 g; Fiber 3 g; Protein 42 g