To prepare the sauce, add the parsley, cilantro, oregano, garlic, Dijon mustard, sherry vinegar, crushed red pepper, 2 teaspoons of kosher salt, and 1 teaspoon of ground black pepper to the bowl of a food processor. Pulse a few times to chop. Add the olive oil and run the processor until the mixture is well-blended. Divide and reserve half the mixture for serving.
To prepare the marinade, whisk half of the Chimichurri sauce with the paprika, chili powder, onion powder, coriander, cumin, oregano, allspice, the remaining teaspoon of kosher salt, and 1/2 teaspoon of black pepper in a medium-size bowl. Place the flank steak in a shallow casserole dish and pour the mixture over the steak, covering all sides. Cover and marinate for 30 minutes–4 hours in the refrigerator.
To grill the steak, preheat grill over medium-high heat. Oil the grill grate and place the steaks on the grill. Discard the marinade. Grill the meat for 5 minutes per side, or to the desired doneness. Serve with the reserved sauce.
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