Prepare the chimichurri sauce: Add 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 to emulsify. The sauce should have a coarse texture.
Add the avocado: Transfer the chimichurri sauce to a medium bowl.
Dice the avocado into 1/2-inch pieces and gently stir into the sauce.
Cover and refrigerate until ready to use.
Cut the steak: If necessary, cut the skirt steak across the grain into 2 to 3 smaller pieces to fit the pan.
Dry and season the steak: Pat the steak dry with paper towels. Season generously on both sides with salt and pepper.
Heat the pan: Heat a 12-inch cast-iron skillet over high heat for 3 minutes.
Add the olive oil once the pan is hot.
Cook the steak: Using tongs, add the steak to the pan, pressing down to ensure good contact.
Sear the first side until browned, about 2 to 4 minutes, then flip and cook the other side for 2 to 4 minutes for medium-rare or until desired doneness.
Rest the steak: Transfer the steak to a cutting board, cover loosely with foil, and let rest for 10 minutes.
Slice and serve: Slice the steak against the grain into 1/4-inch thick slices.
Serve with the avocado chimichurri sauce either on top or on the side.
Prep Time: 20 minutes | Cooking Time: 10 minutes | Total Time: 30 minutes | Kcal: 520 kcal | Servings: 4 servings