In a large zip-top bag (or in a large bowl), combine the olive oil, garlic, rosemary and thyme.
Trim the skirt steak of any excess fat and cut it crosswise into thirds for easier grilling. Put the steak pieces into the bag (or bowl). Mix the marinade around to evenly coat the meat, and let it marinate at room temperature for 30-60 minutes, or refrigerate it for up to 24 hours.
Preheat an outdoor grill to medium-high heat (600˚F / 315˚C), or preheat a grill pan over a medium-high flame. If the steak has been refrigerated, let it sit at room temperature for at least 15 minutes.
Remove the skirt steak from the marinade, scraping off most of the garlic, and pat dry. Season with salt and pepper.
Arrange the steak pieces on the grill. Cook (covered if using an outdoor grill), until browned with grill marks on the bottom, about 3-4 minutes. Flip and cook until browned on the other side, about 3-4 minutes longer for medium rare (the internal temperature should read 130˚F / 54˚C). If using a grill pan, the steak might take a few minutes longer.
Transfer the steak to a cutting board and tent it lightly with foil. Let it rest for 10 minutes. Thinly slice the meat against the grain (take note that the grain goes crosswise, so you’ll need to slice lengthwise). Transfer the meat to a platter, and drizzle it with some of the salsa verde. Serve the remaining salsa verde at the table.