In a food processor, add the garlic. Pulse until minced. Add fresh parsley and cilantro and pulse until finely chopped. Add remaining chimichurri sauce ingredients and pulse until well combined. Set aside.
For the Steak
Marinate the steak: If the steak is large, about 2 pounds, or too big to fit into your skillet, cut it down the middle now. Make sure to cut against the grain (See Tips section in post content for help). Place steaks in a large baking dish or plastic food storage bag and add ⅓ cup chimichurri. Turn to coat. Marinate for at least 30 minutes, preferably 1-2 hours.
Sear the steaks: Heat a large cast iron skillet over medium-high heat for about 3 minutes. Remove steaks from chimichurri and scrape off excess marinade. Pat dry and sprinkle with a modest amount of salt.
If working with two pieces of steak, pour 1 tablespoon of avocado oil into a cast-iron skillet and let heat about 30 seconds or until shimmering but not smoking. If working with steak in one piece, pour 2 tablespoons avocado oil in and heat.
When the oil is shimmering, use a pair of long tongs and carefully place steaks in skillet. Cook 3-4 minutes on each side, depending on the thickness of the steak. See Notes for details.
Remove steak from skillet and let rest on a cutting board for 5 minutes. Repeat with the second piece of steak, if steak had been cut in two. After resting, cut the steak into ½" slices against the grain. Arrange on a large serving platter and top with fresh chimichurri sauce. Serve with additional chimichurri.
Cook Times (Approximate)
For a thicker piece of flank steak:
3 minutes per side = medium-rare
4 minutes per side = medium
For a thinner piece of flank steak:
About 2 1/2 minutes per side = medium-rare
3 minutes per side = medium
Meat Thermometer Test for Doneness
Medium rare = 130–140°F (55–60°C)
Medium = 140–150°F (60–65°C)
Medium well = 150–160°F (66–69°C)
Well done = 160–170°F (70–75°C)
Touch Test for Steak DonenessIf you don't have a meat thermometer, use your tongs to test for doneness. Gently poke the center of the steak whilst searing with a pair of blunt tongs pressed together.
Rare will feel verysoft.
Medium rare will feel a bit firmer but still tender.