- Spice Rub: Mix all ingredients in a medium bowl. Transfer to an airtight container. DO AHEAD: Spice rub can be made 1 month ahead. Store airtight at room temperature.
- Steak: Preheat oven to 200°C. Set a wire rack inside a large rimmed baking tray. Season steak with salt and 2 tablespoons spice rub, pressing onto all sides of meat and adding more spice rub by the teaspoonful if needed to coat entire surface.
- Heat 2 tablespoons oil in a large cast-iron or other ovenproof frying pan over high heat. When oil begins to shimmer, place steak in skillet (be sure to have fan on high; the rub creates some smoke). Sear steak for 1 minute (any longer and the rub will start to burn). Transfer steak to a plate and carefully drain fat from frying pan.
- Wipe frying pan clean with a paper towel. Heat remaining 2 tablespoons oil in frying pan and sear other side of steak for 1 minute. Add butter, garlic, and herbs to frying pan; cook until butter is foamy. Carefully tip frying pan and, using a large spoon, baste steak repeatedly with butter for 1 minute. Turn steak and baste other side for 1 minute.
- Pile herbs on top of steak, transfer frying pan to oven, and roast until an instant-read thermometer inserted horizontally into center of meat registers 125°C for medium, 15-17 minutes.
- Transfer steak to prepared rack; let rest for 20 minutes. Cut steak from bone, slice, and sprinkle with sea salt.