On a plate, season steak generously with salt and pepper. Let sit up to 30 minutes to let steak come to room temperature.
In a large cast-iron skillet over medium high, heat oil until very hot. Add steak and cook until a dark crust forms on the bottom side, about 6 to 7 minutes.
Flip the steak and cook, undisturbed, 6 minutes more.
Reduce heat to medium low and add butter, garlic, thyme, and rosemary. Continue cooking, using a spoon to baste the steak with the butter in the bottom of the pan and flipping the steak halfway through, until steak is medium-rare (about 120°), about 5 minutes more.
Remove steak to a cutting board and let rest 10 minutes before slicing. Sprinkle with flaky sea salt before serving.