Preheat the broiler or a grill. In a food processor or blender, process the cilantro or parsley, olive oil, lime juice, onion and cumin until smooth. With a sharp knife, make many shallow slashes across both surfaces of the pork. Pour half the cilantro mixture over the pork, spreading it evenly over the meat and working it into the slashes. Flip the pork over and repeat the procedure on the other side. If you have time, let the pork marinate to pick up flavors. Otherwise, place it on a broiler pan and sprinkle with salt and pepper. Broil 3 inches from the heat source for 10 minutes. Flip the meat over and broil until cooked through, about 10 minutes more. Remove pork from the oven and let it rest for 15 minutes. Thinly slice the pork across the grain and serve with pan juices delicious warm or at room temperature; great in salads too.