Make the sauce: Combine vinegar, cilantro, basil, garlic, shallot, pepper, salt, and red pepper flakes in a food processor. Pulse 4 or 5 times. Gradually add olive oil and let the processor run about 30 seconds, or until combined.
Marinade steak: Place steak in a glass 9 by 13. Add half of the sauce to coat the top and bottom of the steak. Cover the pan. Let steak marinate for four hours. Reserve the rest of the sauce for serving.
Cook the steak: Oil a gas grill over medium high heat. Cook steak about 5-6 minutes aside for medium rare. Remove steak. Let rest 10-15 minutes.
To serve: Slice the steak against the grain in small slices. Arrange on a platter. Drizzle the sauce over the steak and serve the rest on the side for dipping.
Notes
Katie’s Tips:*My kids prefer the taste of parsley. I swap the cilantro for Italian flat leaf parsley.*Be sure to allow time for the meat to restbefore slicing. This will help keep the meat moist and retain its’ juices.*I ask the butcher to trim my skirt steak to saved me time at home.*The chimichurri is great on chicken or shrimp if you have leftovers!