Heat a large, well-seasoned cast iron skillet over high heat until very hot, 5 to 7 minutes.
Meanwhile, pat the steaks dry with a paper towel and brush them lightly with vegetable oil. Combine the fleur de sel and cracked pepper on a plate and roll the steaks in the mixture, pressing lightly to evenly coat all sides.
When the skillet is ready, add the steaks and sear them evenly on all sides for about 2 minutes per side, for a total of 10 minutes.
Top each steak with a tablespoon of butter, if using, and place the skillet in the oven. Cook the steaks until they reach 120 degrees F for rare or 125 degrees F for medium-rare on an instant-read thermometer. (To test the steaks, insert the thermometer sideways to be sure you're actually testing the middle of the steak.)
Remove the steaks to a serving platter, cover tightly with aluminum foil and allow to rest at room temperature for 10 minutes. Serve hot with Roquefort Chive Sauce on the side.
Roquefort chive sauce:
Bring the heavy cream to a boil in a small heavy-bottomed saucepan over medium-high heat. Cook at a low boil, stirring occasionally, until the mixture has become thick and creamy, about 20 minutes. Remove the pan from the heat, add the cheese, salt, pepper and chives and whisk rapidly until the cheese melts.