Clip the immersion circulator to the side of a large container (I prefer to use this sous vide container) and heat to 129F degrees.
Pat the flank steak dry and season on both sides with half the salt and pepper.
Place in a plastic bag and vacuum seal (I like to use a Foodsaver for the best results). You can also use a ziplock bag and slowly drop it in the water while sealing to get it airtight.
Once the water is heated to the correct temperature, place the bag in the water. It should sink to the bottom, but if it floats, clip the bag to the side to ensure the meat stays fully submerged in the water.
Cook for 2 hours.
Remove from the water and plunge the bag into an ice bath for 10 minutes.
Remove the beef from the bag and pat dry with paper towels. Season again with the remaining salt and pepper.
Heat a skillet over high heat and add a high smoke point oil (ghee is my favorite, but you can use vegetable oil, avocado oil or coconut oil).
Add the meat and sear for 2 minutes per side, until a brown crust has formed on each side.
Remove from the skillet and let it rest for 10 minutes. Slice it against the grain and serve it with the peppercorn sauce (below)