in a cast iron (or stainless steel) pan over medium-high heat, add oil. when hot and shimmering, add flank steak with tongs and allow to sear for 2 and 1/2 minutes on the first side. flip steak and sear for an additional 2 and 1/2 minutes. for a 1- 1/4 " flank steak (like pictured) this will give you a medium-rare flank steak.