Build up a bed of coals and set up two-zone cooking: a hot side and cool side. If using a wood pellet grill, set temperature to 225°F.
Rub steaks with a light coating of olive oil and season both sides of steak with the tandoori seasoning.
Place steak on the cool side of the grill and cook for 30 to 45 minutes or until the thickest part of the meat reaches 120°F. If you're cooking over coals, monitor the temperature and flip and move the steak to avoid overcooking.
Let the steak rest while you heat a cast iron pan on the campfire. Add the butter and sprigs of rosemary to the cast iron pan.
Add steak to the pan and cook the first side for 2 to 4 minutes or until browned.
Flip the steak and cook the other side. Cook while tilting the pan and continuously spooning butter over the steak. Once both sides are browned and braised with butter and rosemary, pull the steak off the pan. Allow to rest for 10 minutes before eating.
Tandoori Spice Rub
Mix ingredients together.