Flank steak is one of my favorite cuts to sous vide. This lean, sometimes a little on the tough side steak benefits from a low and slow cook and a hot sear. I love to do an herby salt to season this steak, but a marinade is also a great way to go.
Preheat water bath using immersion circulator to your desired temperature. 123-127 F for rare, 128-134 F for medium rare, 135-140 F for medium, 141-150 F for well done. I prefer 129 degrees F.
Using a sharp pairing knife, score the flank steak by dragging the tip of the knife diagonally across the surface of the steak in one direction, then in the opposite direction. Repeat on the other side of the flank steak.
When the steak is done cooking, remove the bag from the water bath and the steak from the bag. Using paper towels or a clean dish cloth, pat it dry on both sides.