1. Preheat your oven to 400 degrees, and heat your cast-iron skillet over high heat for 5-10 minutes, until very hot.  (Make sure you’ve got potholders handy… that handle gets dangerously hot!)
2. Mix your fleur de sel and black pepper on a plate, and then press the steaks lightly into the spices to coat all sides of the filets. Once the cast-iron pan is very hot, sear all sides of your filets, about 2 minutes per surface (including sides).  Note: I set off my smoke alarm darn near every time I do this… make sure you’ve got your hood fan going!  It usually takes me about 10 minutes to fully sear my steaks.
3. Once you’ve seared the meat, remove (with your potholder!) the pan from the heat, lay the filets flat, and drop a Tbs. of grass-fed butter on top of each (or don’t!  totally optional). Pop the cast-iron into the oven and cook filets until the desired doneness.  Should take about 7-10 minutes for rare (120 degrees) and a bit longer for medium rare (125 degrees).  
4. Once you’ve reached your desired doneness, remove the pan from the oven, transfer steaks to a plate, cover with aluminum foil, and let those puppies rest for 5-10 minutes. Please. It really makes a difference.

Notes