Remove the flank steak from the refrigerator 30-40 minutes prior to cooking. Pat dry and season well with kosher salt and pepper.
Heat a large cast iron skillet over high hight. (alternatively, heat a gas or charcoal grill to high heat and clean the grates well)
Sear the flank steak 2-3 minutes on each side. Remove the pan from the heat and allow the meat to continue to cook for another 5-6 minutes using residual heat.
Check for doneness: Insert a meat thermometer into the thickest part of the steak - 120°F for very rare, 125°F for rare, or 130°F for medium rare. Flank steak should be served rare or medium rare, otherwise it may be too dry.
If the steak isn't done enough to your liking, return the steak and pan to medium high heat for a few minutes.
Remove the steak from the pan to a cutting board and let rest for 10 minutes, covered with aluminum foil.
Cut the meat into very thin slices, at an angle, across the grain of the meat. (This way you break through the tough long muscle fibers.)