Following manufacturer's instructions, prepare a water bath for 110° - 125°F.
Season filets on both sides with a pinch of salt and pepper.
Place filets in a sealable bag, putting a pad of butter and a sprig of rosemary on either side of the steak.
Submerge sealed bags into a water bath and allow water temperature to return to the desired preset temperature. You may need to clip the bags to your container in order to keep them submerged. I use these cheap ones from amazon and they work just fine.
Allow steaks to sit in a water bath for 1.5 to 2 hours.
When the steaks are done in the sous vide, remove the bags from the water bath, take the steaks out of the bags and pat dry with a paper towel. Set aside.
Prepare your grill for high direct heat, or a cast iron skillet over high heat heat it up to 450° - 500° F. I highly recommend checking your surface temperature with an infrared thermometer if you have one. If you don’t, let the pan preheat over high heat for five minutes or so. This should achieve the temperature you’re looking for. Lightly coat the bottom of the pan with avocado oil.
Place steaks directly onto a hot cast iron surface and sear for 30 seconds. Flip the steaks, placing them in a different spot of the pan on each flip to ensure the hottest possible surface. Repeat for a total of 3-5 minutes, until desired crust has formed.
Remove steaks from pan, top with blue cheese or this amazing steak butter, serve hot.