Add all the ingredients for the chimichurri to a mortar and pestle or a blender. Grind/blend until the desired consistency is reached. Add salt to taste.
Steak
Season steak with salt and pepper generously. Let the steak come to room temperature before cooking.
Lightly grease the cast iron skillet with olive oil. Once the olive oil is hot, add the steak to the pan. Gently press down on the steak so that the entire bottom is touching the pan. Sear on medium high heat for 5 minutes. Use tongs to turn the steak over. Sear the other side for 5 minutes.
Flank and Skirt steaks are best served medium rare, so aim to have an internal temperature of 120 degrees Fahrenheit.
Once the steak is done searing, remove form heat and let rest for 10 minutes. Slice the steak across the grain. Transfer to a serving plate and spoon the chimichurri sauce over.