Juicy, bold-flavored skirt or flank steak marinated in a smoky citrus-chipotle blend, seared hard on a cast iron, and served with charred peppers and onions.
Combine all marinade ingredients in a bowl or zip-top bag and mix well. Add the steak, making sure it's fully coated. Marinate in the refrigerator for at least 1 hour, ideally 4 to 8 hours for the best flavor.
Note: Do not marinate for more than 12 hours, as the citrus and salt can begin to break down the texture of the meat.
About 20 to 30 minutes before cooking, remove the steak from the refrigerator and let it come closer to room temperature. Remove it from the marinade and pat it thoroughly dry with paper towels. Discard the marinade.
Heat a cast iron skillet or heavy-bottomed pan over high heat until it is very hot. Add a small drizzle of neutral oil with a high smoke point (such as avocado or vegetable oil) and swirl to coat.
Lay the steak flat in the hot pan — do not move it. Sear until a deep brown crust forms: about 2 to 4 minutes per side for skirt steak, or 4 to 6 minutes per side for flank steak.
Transfer the steak to a cutting board and let it rest for 10 minutes before slicing.
While the steak rests, return the same pan to medium-high heat (no need to wash it — the leftover fond adds flavor). Add the oil, then the sliced onions and peppers. Season with salt, pepper, and cumin. Cook, stirring occasionally, for 5 to 8 minutes until softened, slightly caramelized, and beginning to char at the edges.
Slice the rested steak thinly against the grain — look for the direction the muscle fibers run and cut perpendicular to them. This shortens the fibers and makes each bite tender. If desired, add the sliced beef back to the pan with the peppers and onions over medium heat for 1 to 2 minutes to bring everything up to temperature together before serving.
To assemble, warm your tortillas directly over a gas flame, in a dry skillet, or wrapped in a damp paper towel in the microwave. Layer steak, peppers, and onions onto each tortilla and finish with your choice of toppings — fresh cilantro, a squeeze of lime, sour cream, salsa, guacamole, shredded cheese, or pickled red onions. Serve immediately.