To prepare the chimichurri, mix the parsley, garlic, salt and red pepper flakes. Add the red wine vinegar and olive oil and mix well.
Cut the avocado into a ½" 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 better fit 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 skillet over high heat for 3 minutes and add the oil once hot.
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 55°C.
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" thick slices. Transfer skirt steak to a serving plate and top with the avocado chimichurri.