Prepare the chimichurri sauce: in a medium bowl, mix the parsley, olive oil, lemon juice, garlic, red pepper flakes and 1/2 teaspoon kosher salt. Set aside.
Season the skirt steak with 1 teaspoon kosher salt and with black pepper. Oil your grill.
Grill the skirt steak until charred on the outside and medium rare on the inside, about 3 minutes per side. On my dual-contact George Foreman grill, I grill the steak for 1 ½ minutes on each side, so a total of 3 minutes. I do flip it midway through grilling, even on a dual contact grill, because the bottom plate tends to be hotter than the upper one.
Transfer the steak to a cutting board, loosely cover with foil, and rest for 5 minutes.
Slice the grilled skirt steak thinly across the grain. Spoon the chimichurri sauce on top and serve.
Consuming undercooked meat may increase your risk of foodborne illness. I always cook lamb and beef steaks, chops and roasts to medium-rare. But you should make your own decision.