Prepare the chimichurri sauce by placing the onion and garlic in food processor. Process until finely chopped. Add the parsley, oil, lime juice, vinegar, salt, pepper and cayenne and process until smooth. Place the steak in a sided dish just large enough to hold it. Pour 1/2 cup of the chimichurri sauce over top. Turn the steak to coat, then cover and marinate, turning occasionally, in the fridge at least 4 hours, or overnight. Pour the remaining sauce into jar. Cover and refrigerate to serve with steak. Preheat the barbecue to high heat. Grill the steak 3-4 minutes per side for medium rare. Transfer the steak to a serving platter and let stand 5 minutes. Cut crosswise into thin slices. Serve remaining chimichurri sauce alongside for spooning on top.