Prep the steak. Take the steaks out of the refrigerator 30 minutes before cooking. Pat them completely dry and season all over with the kosher salt and pepper.
Cut the frites. Peel the potatoes if you like and cut them into ¼-inch sticks. Rinse in cold water until the water runs clear, or soak for 30 minutes, then dry them thoroughly with a kitchen towel.
First fry. Heat the frying oil in a large, heavy pot to 325°F. Fry the potatoes in 2 batches for 5 to 6 minutes, until tender and pale golden. Drain on a wire rack and let rest for at least 10 minutes.
Sear the steak. Heat 2 tablespoons of oil in a cast iron skillet over high heat until shimmering. Sear the steaks for 3 to 4 minutes per side for medium-rare (130°F), adjusting for thickness.
Baste. Lower the heat to medium and add the butter, garlic and herbs. Tilt the pan and spoon the foaming butter over the steaks for about 1 minute.
Rest the steak. Move the steaks to a cutting board and rest for 5 to 10 minutes so the juices settle.
Second fry. While the steak rests, raise the oil to 375°F and fry the potatoes in batches for 2 to 3 minutes, until crisp and deep golden. Drain and salt immediately.
Slice and serve. Slice the steaks against the grain and serve with the hot frites and your favorite sauce.