For a charcoal grill, fill a chimney full of charcoal. When all charcoal hot and covered with ash, pour out and spread coals evenly over half of coal grate. Set cooking grate in place, cover grill, and preheat 5 minutes.
For a gas grill, set half burners to high heat.
Brush grill grate with oil.
Set steaks on cooler side of grill and let cook uncovered, turning occasionally, until an instant-read thermometer registers 105°F (41°C) for rare, 115°F (46°C) for medium-rare, 125°F (52°C) for medium, or 135°F (57°C) for medium-well. Keep an eye on the internal temperature—a lot of factors can impact cooking time.
To sear on grill, transfer steaks to a plate or platter. For a charcoal grill, build up biggest fire possible. If using a gas grill, turn all burners to high heat; preheat grill for 3 minutes.
Return steaks to hottest part of grill and cook, turning frequently, until crisp and charred all over, about 2 minutes total. Let rest 2 minutes then serve.
To sear in cast-iron, about 5 minutes before steaks come off grill, place a large cast-iron skillet on hot side of grill. Once steaks are removed from grill, add 1 tablespoons butter and 1 tablespoon oil to hot skillet; swirling to melt butter and coat skillet.
Add steaks, garlic, and rosemary (if using) to skillet and cook until each side is well-browned, about 1 ½ minutes per side.
Using tongs, hold steaks upright on their sides to sear. Transfer steaks to a cutting board and let rest 2 minutes. Slice and enjoy immediately.