Pre-heat the sous vide water bath: 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 more or less, see the note below*. (It's best to place the container on a cutting board or pad to prevent it from burning the surface of your counter.)
Season the steak: Rub the steak with 1/2 tablespoon oil on all sides.
Season both sides with salt, pepper, minced garlic and optional herbs.
Sous Vide the steak: Add the seasoned sirloin steak to a zip-lock bag. (You can also use vacuum seal bags if you have a vacuum sealer.)
Seal all but one corner of the bag, and slowly place it into the water bath. Make sure everything below the zip-line is covered by water. Then seal the rest of the bag. (Use a kitchen tong to help to push down the bag if the water is too hot.)
Leave the bag in the water bath and make sure it is fully submerged.
Cook for 1 hour for a 1-inch sirloin steak. (It takes 1.5 hours for 1.5 inches, and 2 hours for 2 inches. )
Sear the steak: 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 ziplock bag and wipe off the moisture with paper towels. (It's very important to dry the surface completely, otherwise, it will interfere with the searing.)
Season with more salt and pepper if necessary.
Place the cast-iron skillet on medium-high heat. Add the remaining olive oil. Once hot, add the steak and sear about 1 minute per side. (You can also quickly sear the steak on the grill.)
Slice against the grain and serve with potatoes and veggies.