1/4 cup chopped fresh cilantro
3 tablespoons olive oil
2 tablespoons freshly squeezed lime juice
1 serrano pepper, seeded and finely chopped
2 garlic cloves, minced
1 teaspoon ground cumin
1 pound skirt or flank steak
1 large onion, sliced in half, then cut into strips (with the grain)
2 large bell peppers, cut into strips lengthwise
Tortillas for serving
In a bowl, combine the cilantro, 2 tablespoons of the olive oil, the lime juice, serrano, garlic, and cumin. Add the steak and toss until coated. Marinate in the refrigerator for 15 minutes or overnight.
Heat the remaining 1 tablespoon of the oil in a large cast-iron pan over high heat. Add the steak, frying on each side to your desired doneness (if you want it cooked medium-rare, cook for about 3 minutes on each side). Remove the meat from the pan and set aside on a cutting board, while the vegetables cook.
Reduce the heat to medium-high, add the onion and bell peppers, and sauté until the onion is translucent, about 5 minutes. Remove from the heat but leave the vegetables in the pan.
Cut the meat against the grain into thin slices. Put it back in the pan with the vegetables and serve immediately with warm tortillas.