Prepare water bath for 125°F.
Season filets on both sides with a pinch of salt.
Place filets in sealable bag, putting a pad of butter and a sprig of rosemary on either side of the steak.
Submerge sealed bags into water bath and allow water temperature to return to the selected 125°F.
Allow steaks to sit in water bath for 1.5 to 2 hours before removing.
Just before steaks are done in the sous vide, prepare your cast iron skillet and heat it up as hot as possible, 700°F if possible. Slice off a small piece of beef fat and let it render in the pan. We're going to cook the steak in its own fat not only for flavor, but also at this temperature most other oils will burn.
Remove bags from water bath, take steaks out and pat dry with a paper towel.
Place steaks directly onto hot cast iron surface and sear for 45 - 60 seconds, flip and repeat.
Remove steaks from pan, let rest for 5 minutes before slicing. To put this recipe into overdrive, try topping the steaks with some herb butter.