Thirty minutes to an hour before you are ready to start cooking, remove the filet mignon from the fridge. Pat dry with paper towels to remove the excess moisture, and sprinkle them liberally on all sides.
Line a baking sheet with parchment paper and place a wire rack* over it. Transfer the filet mignon steaks to the wire rack. Loosely cover it with stretch film.
While the meat is resting, preheat the oven to 250 degrees F/121 C.
Transfer the filet mignon to the oven and bake until it reaches your desired doneness*. For a rare steak ( doneness at 120 degrees F), you’ll need to cook for about 35 minutes; for a medium-rare steak (doneness at 125 degrees F), it would take about 40 minutes for the meat to reach the ideal temperature.
Remove the steaks and let rest, uncovered on the counter ((while the cast iron skillet is heating).
Meanwhile, heat a large cast iron skillet over high heat for 5-10 minutes or until very hot. You will know it is hot enough when you splash a drop of water into the pan, and it sizzles, dances around, and evaporates rapidly.
Add the oil to the hot pan and sear the steaks for 45 seconds on each side (including the sides of the steak) until a nice brown crust forms.
At this point, you can remove the steaks from the pan and slice it right away. There is no need to let reverse seared filet mignon rest.
Optionally, in the last few seconds of searing, you can add a tablespoon of ghee or butter and a few sprigs of fresh herbs into the skillet and spoon the melted oil (baste) over the meat for 10 seconds.
Transfer the steaks onto a platter, or alternatively, slice them against the grain and pour over any excess juices from the pan before serving.