Combine olive oil, 4 cloves minced garlic and 2 tablespoons rosemary in a bowl; rub all over steak. Season steak with salt and pepper to taste.
Preheat a large cast-iron skillet or a grill pan over medium-high heat. Add steak, and cook 6-8 minutes per side or to desired doneness. Let stand 10 minutes before slicing.
In a small bowl, mash together butter, 2 tablespoons rosemary, 2 cloves minced garlic and a pinch of pepper. Serve on steak.