Roughly chop 3 of the scallions. Set the remaining 6 aside. In a blender or food processor fitted with the steel blade, combine the basil, parsley, chopped scallions, jalapeño pepper, garlic, vinegar, salt, and pepper. Process or blend in pulses a few times to mix the ingredients. Add the olive oil and process or blend until the mixture turns into a paste.

Pat the skirt steak dry with paper towels and slather the paste all over the steak. Place on a platter or baking sheet and cover with plastic wrap. (Alternatively, you can place the steak in a large food storage bag.) Let sit at room temperature for 30 minutes or for up to 24 hours in the refrigerator. If refrigerated, let sit at room temperature for 15 minutes before grilling.

Brush the grill with some vegetable oil and preheat to medium, about 350-400ºF.

Using paper towels, pat the steaks dry while trying to leave on some of the herb paste. Place the steak over direct heat, close the lid, and grill for 4-6 minutes per side for medium-rare. Remove from the grill and let rest for 5 minutes before slicing against the grain in thin slices.

Brush the whole scallions with some oil. Place on the grill over direct heat and cook for 2-3 minutes per side while the meat rests. Garnish the steak with the grilled scallions and serve.