In a food processor, pulse together the roasted red peppers, red onion, garlic, lemon juice, red wine vinegar, and red pepper flakes. Add the parsley, cilantro, mint, and basil and pulse several more times to combine. With the processor running, add the olive oil in a slow, steady stream, blending until smooth. Season to taste with salt and pepper. Feel free to add more herbs, vinegar, or red pepper flakes according to your taste buds.
Cover the chimichurri and let it sit at room temperature for 1 hour before serving, so that the flavors have time to combine. This can be stored in an airtight container in the refrigerator, for up to 5 days.
Place the skirt steak in a shallow (nonmetal) baking dish. Sprinkle with cumin and garlic, and drizzle with lime juice and 2 tablespoons of the olive oil. Using your hands, rub the seasoning into the meat until it’s evenly coated. Cover and let sit for 30 minutes at room temperature or up to 2 hours in the refrigerator.
Brush the remaining 1 tablespoon of oil on a grill pan and heat over high heat until it starts to smoke. Pat the steaks dry and season on both sides with salt and pepper. Grill about 3–4 minutes per side for medium rare. (I don’t recommend cooking skirt steak much longer or it will get tough.) The steaks should be rosy on the inside with a nice dark crust on the outside. Let the steaks rest for 5–10 minutes (preferably on a plate to catch the juices) then transfer to a cutting board.
To serve, thinly slice the meat against the grain and arrange on a platter, drizzled with the extra juices. Place the chimichurri in a bowl alongside.