In a mixing bowl, combine the vinegar, salt, pepper, garlic, shallot, and jalapeño. Next, mix in the cilantro and parsley. Then using fork, lightly whisk in the oil.

Marinate the steak using about 1/3 to 1/2 of the marinate for about 3 hours in the refrigerator and up to overnight. Store the remaining chimichurri sauce in an air-tight container at room temperature.

Using a cast iron grill pan on high heat, sear on each side for about 3 minutes.

After searing each side, turn off the heat and let the steak continue to cook for approximately 10 minutes, 5 minutes on each side. Using your meat thermometer, take the steak off the pan when it hits 130 degrees F for medium rare (140 for medium, 150 for well-done). For a flank steak, I highly recommend medium-rare. Anything else will be quite tough.

 When the steak reaches the desired temperature, remove it from the pan and let it rest on a cutting board, cover with aluminum foil for about 8-10 minutes before slicing. Tip: Slice the steak at a downward, 45 degree angle to the cutting board, which will help tenderize the steak.