How to Make It
In a food processor, combine the parsley and garlic with the 3/4 cup of olive oil and the vinegar and pulse until smooth. Refrigerate the chimichurri for at least 2 hours or up to 8 hours.
Using a sharp chef’s knife, make a 1/4-inch, lengthwise cut in the top of the tenderloin. Turning the tenderloin and rolling it out as you go, spiral-cut the meat until you have a long, rectangular piece about 1/4 inch thick.
Light a grill. Season both sides of the tenderloin with salt and pepper. Rub all but one-third of the chimichurri over the meat and grill over moderately high heat, turning once, about 4 minutes for medium-rare meat; let rest for 5 minutes before slicing.
Meanwhile, in a bowl, mix the remaining chimichurri and olive oil. Season with salt and pepper and serve with the steak.