Preheat oven to 450°F. Season steaks with salt and pepper.
Heat an oven-safe or cast-iron skillet over medium-high heat. Place the butter and oil into the pan. When the butter has melted, place steaks into the pan and sear for 2 minutes on each side, or until they are nice and golden brown. Toss in the rosemary and garlic, and place in the heated oven for about 7-9 minutes—7 minutes usually makes for a perfect medium!