Fill a sous vide container or a large pot with water. Attach your sous vide precision cooker and set the temperature according to your desired level of doneness. For flat iron steak, 130°F (54°C) for medium-rare or 135°F (57°C) for medium is recommended.
Season the flat iron steak generously with salt and pepper. Optionally, you can add minced garlic, fresh herbs, or a drizzle of olive oil for extra flavor.
Place the seasoned flat iron steak in a vacuum-sealable bag or a sturdy, food-grade zipper-lock bag. Remove as much air as possible from the bag using a vacuum sealer or the water displacement method (submerge the bag in water, leaving the top open, and seal the bag once most of the air is removed).
Once the water bath reaches the desired temperature, carefully lower the sealed bag into the water. Ensure the steak is fully submerged. Cook the flat iron steak in the sous vide water bath for 2-4 hours. The longer cooking time will result in a more tender steak, but even a couple of hours will yield excellent results.
Optional: After sous vide cooking, remove the steak from the bag and pat it dry with paper towels. Preheat a skillet or grill over high heat. Sear the steak for 1-2 minutes on each side until a golden crust forms. This step is optional but adds additional flavor and texture to the steak.
Once seared (if desired), transfer the flat iron steak to a cutting board and let it rest for a few minutes. This allows the juices to redistribute evenly throughout the steak. Slice the steak against the grain and serve immediately.
