Set your sous vide water bath to 135-138 degrees - the temperature for medium rare.
Liberally add salt and fresh pepper to both sides of the steak to your tastes.
Seal the steak with a vacuum sealer or the water displacement method.
Add to water bath.
Cook for 1-2 hours.
Remove the steak from the water bath. I use tongs so not to touch hot water.
Put a cast iron skillet over high heat. Once the skillet is super hot, add 2 tablespoons of your favorite cooking oil. Cook the steak for 30 seconds to 1 minute per side to brown each side.
Slice the steak against the grain when serving.
Notes
Find that your steak is floating? Clip it to the side of the water bath or use a heavy object like a pot to weigh it down. Make sure to not pass the max fill line for your machine.
When your steak is done, you can drain the liquid (au jus) inside the bag into a small bowl for dipping. It's optional.