Place steak in a zip lock bag along with rosemary, thyme and the minced garlic. Partially close the sealer, leaving a few inches open.
Slowly submerge the bag with the steak in a water filled container, deep enough to cover the entire steak, but do not let any water enter it. Pressure will force the air out of the bag, and seal it once the air is all out.
Fill a large pot with water and place the thermometer in it. Place the pot over medium heat. Once you reach 135F, submerge the bag of steak inside the pot and leave it to cook for 90 minutes, for medium doneness. Be sure to regularly check the temperature, allowing a 50F buffer but no more.
Once cooked, remove the steak from ther bag. Heat butter in the iron cast skillet until it smokes and place the steak on top. Sear both sides and serve immediately.
Notes
TO STORE: Put the steak in airtight containers and store for 3-4 days.TO FREEZE: Use freezer-safe bags to freeze steak for up to 2 months.TO REHEAT: Reheat in an oven or skillet.