
Make the marinade: In a medium bowl, whisk together all of the ingredients except for the skirt steak.
Marinate in the fridge: Place the skirt steak in a large baking dish or gallon size zip lock bag and pour the marinade on top. Turn the steak a few times until it's completely coated with marinade and refrigerate for 2 to 6 hours. Do not marinate for longer than 8 hours because the meat will start to break down.
Preheat grill to medium-high heat (about 400°F-450°F) for direct-heat grilling.(For gas grilling, simply light all the burners on high, close the lid and let the grill heat up for 5-10 minutes.) (For charcoal grilling, the process is a bit more involved but the flavor is worth it. Here's a super-quick read with step-by-step photos on how to start a charcoal grill.)
Prepare the steak: Remove steak from marinade and discard any excess marinade. Pat steak dry with paper towels and season lightly with coarse salt and black pepper on both sides.
Grill the steak: Grill the skirt steak with the lid closed for about 3-4 minutes per side for medium rare, or longer depending on the thickness and your desired doneness.
Rest and Slice: Transfer the grilled steak to a cutting board and let it rest for 5 minutes. Slice thinly across the grain and serve with cilantro pesto.
Add all ingredients to a food processor or blender and pulse until mostly smooth.