In a food processor combine the garlic, jalapeño, and onion and pulse until finely chopped. Then add the cilantro, parsley, vinegar, lemon juice, oregano, salt, pepper, and crushed red pepper flakes and pulse until finely chopped. Gradually add the oil in a steady stream through the feed tube and process until well blended but some texture still remains, about 1 minute. Reserve ¾ cup chimichurri for the marinade. Transfer the remaining chimichurri to a small bowl for serving. Cover and refrigerate. Allow the chimichurri to stand at room temperature for 30 minutes before serving.
Place the steak pieces in a large resealable plastic bag and pour in the ¾ cup reserved chimichurri. Press the air out of the bag and seal tightly. Turn the bag and massage the chimichurri into the meat. Place the bag in a bowl and refrigerate for 2 to 4 hours, turning and massaging occasionally. Allow the steaks to stand at room temperature for 15 to 30 minutes before grilling.
Prepare the grill for direct cooking over high heat (450° to 550°F).
Remove the steaks from the bag and season evenly with the salt and pepper. Discard the chimichurri in the bag. Brush the cooking grates clean. Grill the steaks over direct high heat, with the lid closed, until cooked to your desired doneness, 4 to 6 minutes for medium rare, turning once or twice. Remove from the grill and let rest for 3 to 5 minutes. Cut the meat across the grain into thin slices and serve warm with the remaining chimichurri.