Add the cilantro, parsley, garlic, salt, cumin, and red pepper flakes to a food processor. Pulse until the leaves are coarsely chopped, about 5 pulses.

Add the red wine vinegar, pulse 5 times.

Add the olive oil, pulse 5 times. The chimichurri sauce should be emulsified and the herbs coarsely chopped.

Transfer the chimichurri sauce to a medium bowl. Cut the avocado into ½-inch dice. Add the avocado to the chimichurri and gently stir to combine. Cover and refrigerate until ready to use.

If the skirt steak comes in a long strip, cut across the grain into 2 to 3 smaller pieces, about 5 to 7 inches in length. This will allow the steak to fit better in the pan.

Thoroughly pat dry each steak with paper towels on each side. The surface should be very dry to help brown the surface of the meat. Generously season each side of the steaks with salt and pepper.

Heat a large 12-inch cast-iron skillet over high heat for 3 minutes. Once hot add the olive oil.

Use tongs to carefully add the steak to the pan, then press down on the surface a few times for more contact. Sear the first side until browned, about 2 to 4 minutes depending on the thickness of the meat. Flip and cook another 2 to 4 minutes. For medium-rare doneness, cook steak until it reaches an internal temperature of 130°F. Work in two batches if needed.

Transfer steak to a cutting board, loosely cover with foil and rest for 10 minutes before slicing.

Hold the carving knife at a 45-degree angle and then slice the steak against the grain, about ¼-inch thick slices. Transfer skirt steak to a serving plate and either top with avocado chimichurri sauce or put the sauce on the side.