Season the steak well with salt and pepper. Let sit at room temperature for at least 1 hour or refrigerate uncovered for up to 48 hours. Meanwhile, make the chimichurri: In a blender, purée the parsley, cilantro, oil, vinegar, garlic, jalapeño, 1 tsp. salt, and 1/4 tsp. pepper, adding more oil as needed. Transfer to a small bowl and season to taste with more salt and vinegar, if needed.
If the steak is refrigerated, let it come to room temperature before grilling. For a charcoal grill, light a chimney starter full of charcoal and bank the coals against one side of the grill. For a gas grill, start with all burners on medium high and then turn off one or more of the burners to create a hot zone and a cool zone.
Pat the steak dry and grill over the hottest part of the grill, rotating it occasionally to create a crust, until well browned on one side, 2 to 4 minutes. Flip and sear the other side.
Move the steak to the cooler part of the grill, cover, and cook to your desired doneness (about 14 to 18 minutes for medium-rare). Let rest 5 to 10 minutes before serving with the chimichurri sauce.
Make Ahead Tips
The sauce will keep in the fridge for 1 week.
Photo: Scott Phillips