Add water to the sous vide container or a large pot, set the Sous Vide Precision Cooker to (135°F/57°C) for medium rare doneness. If you’d like to cook it less, see the note below*.
Rub the filet mignon with 1/2 tablespoon oil on all sides.
Season with salt and pepper.
Add the steak to a zip-top bag together with 1/2 tablespoon butter and optional herbs.
Vacuum seal the bag using water displacement method: seal all but one corner of the bag, and slowly place it into cold water. Make sure everything below the zip-line is covered by water. Then seal the rest of the bag.
Place the bag into the water bath in the sous vide container. Make sure it is fully submerged. (If your meat is floating, you can use heavy kitchen items such as metal kitchen tongs to weigh it down.)
Cook for 1 to 4 hours depending on the thickness of your filet mignon. Cook 1-inch thick steak for 1 hour, 1.5-inch thick for 2 hours, 2-inch thick for 2.5 hours, and 2.5-inch for 4 hours.
When the timer goes off, remove the bag from the water bath and transfer it to the fridge or an ice bath. Let it chill for about 10 minutes.
Remove the steak from the zip-top bag and wipe off the moisture with paper towels. (Make sure to pat dry the surface completely.)
Season with more salt and pepper if necessary.
Place the cast-iron skillet on medium-high heat. Add the remaining olive oil. Once very hot, add the remaining butter and let it melt. Add minced garlic, herbs, and the steak and sear about 1 minute per side until nicely browned.
Slice against the grain and serve immediately.