Preheat the oven to 450 degrees F. Whisk the mustard and olive oil in a small bowl. Coat the steak with 2 tablespoons of the mustard mixture, then season with black pepper and set aside. Whisk the chives, tarragon and 2 tablespoons cold water into the remaining mustard mixture. Slice the potatoes into 1/4-inch-thick sticks; pat dry. Whisk the egg whites, paprika, cayenne and 1/4 teaspoon salt in a large bowl; add the potatoes and toss. Coat a rimmed baking sheet with cooking spray. Add the potatoes, letting the excess egg drip off; spread out and spray with cooking spray. Bake until golden brown, 25 minutes. Meanwhile, cook the steak: Sprinkle 1/4 teaspoon salt in a large cast-iron skillet. Heat the pan over high heat until very hot, then add the steak and place another heavy skillet on top to weigh it down. Sear until browned, about 3 minutes per side. Turn off the heat and let the steak rest in the pan, 5 to 8 more minutes for medium rare. Thinly slice against the grain and serve with the fries and herb mustard.