Set the sous vide cooker at 57°C / 135°F to preheat the water. This is the right temperature for beef to get that perfect pink center.
Place the beef in the vacuum seal bag, add the rosemary, and the vegetable seed oil. Add the garlic clove and seal the bag.
When the water is heated, place the bag into the sous vide container. Cook for 16 hours.
Once the beef has finished cooking, take it out of the vacuum sealed bag.
Discard the whole liquids, and dry the meat down.
Melt and heat the butter in the cast iron skillet.
Place the roast beef on the hot skillet, and let it roast in the butter until all sides are golden brown.
Season with salt as desired and it's ready to eat. Thinly slice the roast beef and enjoy!