Smear a thin layer of mustard onto all sides of steaks. Mix rosemary, salt, and pepper, and sprinkle over both sides of steaks.
Set Pellet Boss control to HIGH, and preheat until grill reaches 400ºF+. With the flame zone uncovered, sear steaks until internal temperature reaches five degrees below your desired final temperature (see below). The steak’s temperature will continue to rise approximately five degrees (carryover cooking) after removing from the grill.