Remove the filet mignon steaks from the refrigerator 30-60 minutes before cooking to bring them to room temperature. Unwrap the steaks, season generously with salt and ground black pepper on all sides, and let them sit on a plate on the counter. This step ensures the steaks cook evenly and helps develop a better crust; I never skip it for the best results.

Preheat the oven to 360°F (182°C). Place a medium cast iron skillet over high heat for 3-5 minutes until very hot. Carefully add the oil—be cautious as it may splatter—and immediately add the seasoned filet mignon steaks (from Step 1). Sear each side for 2-3 minutes, or 1-2 minutes if using petite filets, until well browned with a nice crust. Once seared, remove the skillet from the heat.

With the steaks still in the skillet, add the unsalted butter, peeled garlic cloves, and fresh rosemary sprigs around the steaks. Carefully transfer the entire skillet to the preheated oven. Roast for 2 to 8 minutes, depending on your preferred level of doneness: about 2 minutes for rare, 3-4 minutes for medium-rare, 5 minutes for medium, 6-7 minutes for medium-well, or up to 8 minutes for well-done. Remove the skillet from the oven when the steaks reach about 5-10°F below your desired final temperature, as they'll continue to cook while resting.

Remove the steaks from the skillet and place them on a plate. Tent loosely with foil and allow them to rest for 5-10 minutes before serving. This resting period helps the juices redistribute, keeping the steaks tender and juicy. Serve with the softened garlic cloves and melted butter from the skillet—it's a finishing touch I always enjoy for both flavor and presentation.