Preheat a grill pan over high heat for 3 to 4 minutes.
Combine the mustard, onion powder, garlic powder, salt, cayenne and allspice in a small bowl. Rub the flank steak all over with the mix and let sit for 10 minutes.
Put the steak on the hot grill and cook 5 minutes per side. Remove and let rest 10 minutes before slicing. Serve with Chimichurri Sauce.
Put the parsley, cilantro, capers and garlic in a medium mixing bowl and toss to combine. Add the vinegar, salt, red and black pepper and stir. Pour in the olive oil and mix until well combined. Let sit for 30 minutes so that the flavors blend.
Per Serving (steak only - does not included sauce): Calories: 329; Total Fat: 12.5 grams; Saturated Fat: 5 grams; Protein: 49 grams; Total carbohydrates: 1 gram; Sugar: 0 grams; Fiber: 0 grams; Cholesterol: 141 milligrams; Sodium: 606 milligrams