Follow these steps to create a mouthwatering filet mignon in cast iron:

1. Bring to Room Temperature: Remove the filet mignon from the refrigerator and let it sit at room temperature for about 30 minutes.
2. Preheat Oven: Preheat your oven to 400°F (200°C).
3. Season the Steaks: Generously season both sides of the filet mignon with salt and pepper.
4. Heat the Skillet: Place a cast iron skillet over medium-high heat and add the olive oil. Allow it to heat until it shimmers.
5. Sear the Steaks: Place the filet mignon in the skillet without crowding. Sear for 3-4 minutes until a golden-brown crust forms.
6. Add Flavor: Flip the steaks and add butter, crushed garlic, and rosemary or thyme to the skillet. Spoon the melted butter over the steaks for enhanced flavor.
7. Transfer to Oven: Once seared, transfer the skillet to the preheated oven. Cook for an additional 5-10 minutes, depending on your desired doneness (125°F for rare, 135°F for medium-rare).
8. Check Temperature: Use a meat thermometer to check the internal temperature. Remove the skillet from the oven when it’s about 5°F below your target temperature, as it will continue to cook while resting.
9. Rest the Steaks: Transfer the filet mignon to a cutting board and let it rest for 5-10 minutes to retain its juices.
10. Serve: Slice and serve the filet mignon, drizzling extra butter from the skillet over the top if desired.