Mix all the ingredients for the marinade together in a shallow dish.
Trim the skirt steak of any chunks of fat, and cut them into manageable pieces, about 6 inches long.
Add the steak to the marinade and refrigerate, covered, for at least 1 hour or as long as 8 hours (see recipe tip).
Toss the onion and bell peppers in the oil, salt, and pepper.
When ready to cook, prepare the grill for 2-Zone Grilling, with the sear zone activated, and the remaining burners on low. Preheat for at least 15 minutes and brush the grill grates clean.
Remove the steak from the marinade and wipe off any excess.
Add the vegetables to the grill above the lower-heat burners and cook, with the lid closed, for 5 minutes. Flip the vegetables and add the steak to the Sear Zone.
Cook the steak for 2 minutes a side for medium-rare, and the vegetables until softened and slightly charred, about 5 minutes on the second side.
Transfer the steak and vegetables to a cutting board and rest for 5 minutes.
While the steak rests, add the tortillas to the grill and cook until soft and pliable, turning constantly, for about 2 minutes.
To serve, first cut the steak into 3"–4" wide pieces. Rotate each piece so that your knife is perpendicular to the direction of the muscle fibers of the steak (also called the “grain” of the steak). Slice the steak into thin slices and transfer to a platter.
Cut the vegetables into thin slices as well and place them on the platter with the steak. Serve the steak and vegetables with the tortillas, cilantro, salsa, sour cream, and any other toppings you’d like.
Recipe Tip
We love marinating skirt steak, because the loose muscle fiber allows the marinade to really soak into the meat. However, if you don’t have the time, substitute the marinade for a tablespoon of your favorite fajita seasoning blend.
