Prepare the chimichurri. Combine the parsley and garlic in a food processor and process until finely chopped. Add the oil, 1/4 cup vinegar, water, 1 1/2 teaspoons salt, and 1 teaspoon pepper and process to make a thick sauce. Correct the seasoning, adding salt, pepper, or vinegar as necessary; the mixture should be highly seasoned. Place half the chimichurri in a bowl or crock for serving; pour the remainder over the meat. Cover and let marinate in the refrigerator, 30 minutes, turning several times.
Preheat the grill to high (or broiler).
When ready to cook, oil the grill grate. Drain the beef and place it on the hot grate. Grill, turning with tongs, until cooked to taste, about 10 minutes per side for medium-rare. Serve with the remaining chimichurri sauce.
It is normally served on a wooden cutting board. I cut it like london broil in thin slices across the grain.