Process 1/2 cup cilantro, vinegar, oil, orange juice and zest, jalapeño, and salt in a food processor until coarsely chopped.
Rub half of the marinade over steak, and refrigerate for 1 hour.
Preheat grill to medium-high. (If you are using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 3 or 4 seconds.) Grill steak, flipping halfway through, until a meat thermometer registers 125 for medium-rare, about 12 minutes. Meanwhile, grill onion, flipping once, until it begins to soften, 3 to 5 minutes.
Let steak rest for 10 minutes before thinly slicing. Serve with onion and remaining 1/2 cup cilantro and marinade, and garnish with parsley.