1. Pat the steak dry with a paper towel to remove any excess moisture, then season generously with salt. Remove the leaves from 1 sprig of the rosemary and add to a small bowl along with the lemon, 4 sprigs of the thyme and 4 cloves of the garlic; mix to combine. Using your hands, rub the steak with the lemon-herb mixture. Wrap the steak in plastic wrap and refrigerate for at least 4 hours but preferably overnight.
2. Remove the steak from the refrigerator and allow to come to room temperature, about 1 hour. Remove and discard the rosemary, thyme, lemon and garlic. Rub the steak with the olive oil.
3. Heat a large cast-iron skillet over medium-high heat until very hot. Add the steak and cook until the bottom is crusty, about 4 minutes. Using tongs, hold the steak up on its sides, turning occasionally, to sear its edges, about 5 minutes. Flip the steak. Add the butter to the skillet along with the remaining rosemary, thyme and garlic. Using a large spoon, baste the steak with the herb butter until medium-rare, or until an instant-read thermometer inserted into its center reaches 130°, about 5 minutes more.
4. Transfer steak to a cutting board and allow to rest for 10 minutes. Cut the steak off the bone and slice the steak, against the grain, into thin slices. Garnish with fleur de sel. Serve immediately.